When defending champion Naomi Osaka and teenage sensation Coco Gauff meet in the third round of the US Open on Saturday, even Serena Williams says she will be “excited to be a fan girl”.

Gauff, 15, became a household name when she reached the fourth round at Wimbledon before losing to eventual champion Simona Halep.

It is only her second appearance in a Grand Slam singles main draw.

“I definitely think it’s the future of women’s tennis,” said Williams.

“I think it’s super-exciting tennis. It will be a really interesting match,” Williams, who lost to Osaka in last year’s final, added.

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Japan’s Osaka, who won the Australian Open in January, has struggled with form and injury since becoming world number one.

The 21-year-old reached just the third round at the French Open before being knocked out at the first stage at Wimbledon and has also failed to beat a top-10-ranked player since the start of the year.

Meanwhile, Gauff has continued her impressive form, winning her first WTA doubles title at the Washington Open alongside compatriot Catherine McNally, 17.

“She’s done so well again here this tournament,” said Swiss five-time champion Roger Federer. “I think we’re all a bit surprised that she’s able to back it up after Wimbledon which was already an incredible run.

“For her to strive in those conditions shows she’s going to have a great future ahead of her. It’s going to be a big day [on Saturday]. I wish her all the best, of course.”

Seeds go head-to-head

Elsewhere in the women’s draw, former Australian Open champion and two-time US Open finalist Caroline Wozniacki is up against Canadian 15th seed Bianca Andreescu on Arthur Ashe.

Nineteen-year-old Andreescu clinched her second WTA title in Toronto this month following Williams’ retirement in the final and the teenager also beat German three-time Grand Slam champion Angelique Kerber in the final at Indian Wells in March.

Dutch seventh seed Kiki Bertens takes on German 26th seed Julia Gorges on Louis Armstrong, while Belgian 25th seed Elise Mertens faces Germany’s Andrea Petkovic, ranked 88th, on Grandstand.

Elsewhere in the men’s draw…

Spanish second seed Rafael Nadal takes on South Korean world number 170 Chung Hyeon in the second match on Arthur Ashe.

Nadal, who was knocked out by Argentine Juan Martin del Potro in last year’s US Open semi-final, is seeking a fourth title in New York.

Australian 28th seed Nick Kyrgios, who lost to Nadal in the second round at Wimbledon, plays Russian Andrey Rublev, ranked 43rd.

And German sixth seed Alexander Zverev, who has failed to go beyond the quarter-finals of a Grand Slam this year, faces Slovenian world number 80 Aljaz Bedene on Louis Armstrong.

Britain’s Dan Evans and Cameron Norrie beat German pair Tim Putz and Jan-Lennard Struff in the first round of the men’s doubles at the US Open.

The Britons, who were knocked out of the singles competition this week, won 6-3 1-6 6-1 on court nine.

Evans had earlier lost 6-2 6-2 6-1 to five-time singles champion Roger Federer on Arthur Ashe.

Compatriot Jonny O’Mara is also through, alongside partner Marcelo Arevalo from El Salvador.

They beat German Mischa Zverev and Frenchman Benoit Paire 4-6 6-4 6-1.

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But Britain’s Ken Skupski, alongside New Zealand partner Marcus Daniell, were beaten 6-7 (5-7) 7-5 7-6 (7-1) by Australian Nick Kyrgios and Romanian Marius Copil.

In the mixed doubles, Britain’s Joe Salisbury and American Desirae Krawczyk booked their place in the second round with a comfortable 6-2 6-2 victory over Americans Hailey Baptiste and Jenson Brooksby.

Meanwhile, Britain’s Neal Skupski and Spanish partner Maria Jose Martinez Sanchez lost 6-4 6-3 to fourth seeds Ivan Dodig and Latisha Chan from Bosnia and Herzegovina and Taiwan respectively.

Elsewhere, Belarusian Victoria Azarenka and Australian Ashleigh Barty, seeded eighth, beat Romanian Monica Niculescu and Russian Margarita Gasparyan 4-6 6-1 6-1 to progress to the second round of the women’s doubles.

And American teenagers Catherine McNally and Coco Gauff – who won the Washington Open together this month – triumphed 7-6 (8-6) 6-2 against German Julia Gorges and Czech Katerina Siniakova.

Gauff, 15, takes on world number one Naomi Osaka in the third round of the women’s singles on Saturday.

Former US Open champion Juan Martin del Potro expects to make his return from a broken kneecap at Stockholm in October.

The Argentine, 30, has not played since hurting his right knee at Queen’s in June, during the build-up to Wimbledon.

Del Potro, who first fractured his kneecap last October at the Shanghai Masters, had surgery before returning home to begin his recovery.

“I’m very happy to tell you that I’ll play in Stockholm and Vienna in October,” he said on Twitter.

“These two tournaments are very special to me, as I had the chance to win both of them.”

Del Potro won the US Open in 2009 and finished runner-up last year to Novak Djokovic, but has missed this year’s tournament because of the injury.

He is a two-time winner of the Stockholm Open, which runs from 14-20 October, and also claimed the 2012 title in Vienna, where he plans to compete the week after.

Third seed Karolina Pliskova is among the favourites to win the women’s singles at the US Open and will play Tunisia’s Ons Jabeur in the third round on Friday.

In her latest BBC Sport column, the Czech discusses her relationship with twin sister Kristyna, who is also a WTA Tour player, how they shared a hotel room before they played each other in Birmingham recently…

When the draw for a Grand Slam tournament is made I’m always worried that Kristyna and I will be drawn against each other in the first round – that would be the worst day of my life!

There would be too much pressure and too many nerves because the Grand Slams are so important and mean so much to us.

So it is always a relief when my sister is in the opposite half.

That’s what happened here at the US Open, although it was sad that she lost to Elise Martens in their second-round match on Thursday.

We don’t watch the draw as it is being made, nor do we follow it on social media, and then when we do eventually see it we message and always say: ‘Phew, we are not playing each other!’

We have only played each other once on the WTA Tour and that came a few months ago in Birmingham.

It was a very difficult situation for both of us – and the worst thing was that we were sharing a hotel room at the tournament!

We have played each other many times when we were younger – in qualifiers and on the lower-ranked tours – but we hadn’t played for about six or seven years.

We had come close a couple of times in main draws but either I lost or she lost earlier, and also I had to withdraw once because of injury when we were supposed to play in Prague.

So it felt like everybody was waiting for this match to happen, the Czech media were excited and many people were talking about the match.

‘Our parents couldn’t watch’

It was a strange atmosphere in our hotel room on the morning of the match.

Kristyna and I were trying to be calm and act normal, but all the people around us and our teams were stressing and wanted to talk about it so much.

Thankfully our parents weren’t there in Birmingham, I can’t imagine how horrible it was for them. They didn’t even watch on television back in the Czech Republic.

My mum doesn’t usually like to watch our matches anyway because she is too nervous. My father usually does watch matches – but not this one. He did something else to take his mind away from it.

So we both felt very nervous and didn’t know what would happen – if we would play good tennis or bad tennis.

It was huge pressure for me because if you look at the rankings I should win easily.

Of course I wanted to win the match, but I also didn’t want my sister to lose. It was an emotional rollercoaster. We ended up playing a long match and Kristyna won 6-2 3-6 7-6 (9-7)!

It was the biggest victory of her career – she had never beaten a top-five player before that.

Kristyna is ranked 86th at the moment but I believe she has the ability to improve that a lot. She is not easy to beat.

If we could both be ranked in the top 10 together that would mean a lot to both of us – even more to her of course – and our families.

But that would also mean we would play each other even more, which is something I’m not sure I would enjoy!

Maybe it is something you could get used to if it is happening more often, and then we wouldn’t be as super stressed about it like we were in Birmingham.

‘Sometimes I talk to Kristyna more than my husband’

Of course we look similar because we are identical twins. However, I think we have aged a little differently, and we try to be different because we have never wanted to be the same.

We have different tattoos. She is a little more blonde. And our personalities are quite different.

But sometimes we think in the same way, and we often say the same thing at the same time – that makes people laugh.

She thinks a little more about stuff and is more responsible. She is two minutes older – so maybe she is trying to be like the boss a little bit and protect me!

We still give each other advice about tennis; we give advice to each other about everything in life.

Barely an hour goes by when we aren’t talking on the phone – I text “hi” as soon as I wake up and and we continue all day on WhatsApp.

Sometimes I speak to her more than my husband! He doesn’t mind though and he’s happy for Kristyna to hang out when it is just the three of us.

The week before the US Open we went to watch some Broadway shows together – Pretty Woman and Frozen – and had a few dinners, and also did some walking and shopping.

With us being in different parts of the draw – I played Monday and Wednesday, and Kristyna played Tuesday – we have not done much in the past three or four days because you don’t want to go into Manhattan and waste energy.

I need all my focus on going as far as I can in the tournament and hopefully winning my first Grand Slam.

Karolina Pliskova was talking to BBC Sport’s Jonathan Jurejko at Flushing Meadows

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German sixth seed Alexander Zverev was taken to a deciding set for a second successive match as he saw off American Frances Tiafoe 6-3 3-6 6-2 2-6 6-3 in the US Open second round.

Zverev hit 52 unforced errors as the momentum swung throughout.

Second seed Rafael Nadal had a walkover, with Australian Thanasi Kokkinakis withdrawing with a right shoulder injury.

The three-time champion will play South Korea’s Chung Hyeon in the next round.

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Australian Nick Kyrgios produced a stunning display of serving as he hit 24 aces in his routine 6-4 6-2 6-4 win over unseeded Frenchman Antoine Hoang.

Kyrgios, the 28th seed, has now reached the third round four times but has never made it past that phase. He now takes on unseeded Russian Andrey Rublev, who got through as a result of the retirement of Frenchman Gilles Simon after the first set of their match.

Meanwhile, Zverev will play Slovenian Aljaz Bedene, who beat 29th seed Benoit Paire, in the third round.

The 22-year-old, who defeated Radu Albot in the first round, said: “Frances gave me an unbelievable fight today. I’m just happy to get the win somehow.”

Elsewhere, in-form Cincinnati Masters champion Daniil Medvedev recorded his 16th win from the past 18 matches with a 6-3 7-5 5-7 6-3 victory over Hugo Dellien of Bolivia. The Russian fifth seed will now play Spain’s Feliciano Lopez.

American Denis Kudla’s reward for defeating 29th seed Dusan Lajovic 7-5 7-5 0-6 6-3 is a third-round contest against defending champion and world number one Novak Djokovic.

French 13th seed Gael Monfils and Canada’s world number 33 Denis Shapovalov will face each other after they enjoyed comfortable wins. Monfils defeated Romania’s Marius Copil 6-3 6-2 6-2 and Shapovalov beat Swiss Henri Laaksonen 6-4 7-6 (7-2) 6-2.

Swiss 2016 champion Stan Wawrinka, 34, saw off fellow veteran Jeremy Chardy of France 6-4 6-3 6-7 (3-7) 6-3. The 23rd seed will now play Italy’s Paolo Lorenzi.

Belgian 15th seed David Goffin, beaten by Medvedev in the recent final at Cincinnati, defeated Gregoire Barrere 6-2 6-2 6-2 and will play Spain’s Pablo Carreno Busta in the third round.

American 14th seed John Isner, a quarter-finalist last year, will play Croatia’s 2014 champion Marin Cilic, after he secured a 6-3 7-6 (7-4) 7-6 (7-5) victory over Jan-Lennard Struff of Germany.

And Australian Alex de Minaur, 20, overcame Chile’s 31st seed Cristian Garin 6-3 7-5 6-3 to set up a tie against seventh seed Kei Nishikori.