Germany’s Alexander Zverev clinched the last spot in the ATP Finals semi-finals by beating John Isner, a result which ensured world number one Novak Djokovic finished top of their group.

Zverev nicked a first-set tie-break before breaking at the end of the second to win 7-6 (7-5) 6-3 in London.

Third seed Zverev will play Roger Federer in the semi-finals on Saturday.

Djokovic, who beat Marin Cilic 7-6 (9-7) 6-2 later on Friday, will play South Africa’s Kevin Anderson.

The 14-time Grand Slam champion is the heavy favourite to win the tournament after easing through the round-robin stage with three victories from his three matches.

In the doubles, Pierre-Hugues Herbert and Nicolas Mahut won their group by beating Mike Bryan and Jack Sock 6-2 6-2 in 53 minutes.

Bryan and Sock finished second and will face Britain’s Jamie Murray and Bruno Soares in the semi-finals.

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Djokovic provides another statement of intent

Djokovic was identified as the man to beat at the O2 following his remarkable rise back to the world number one ranking, with dominant performances against Isner and Zverev sending further warning signs to the rest of the field.

While he was made to work much harder by Cilic in the first set, the Serb produced another impeccable performance to maintain his record of not dropping a set this week.

Even though his passage to the semi-finals was already guaranteed as the group winner, Djokovic refused to let his intensity drop and strangled Cilic with a near-perfect service game.

The Wimbledon and US Open champion only dropped five points on serve in the match and won 100% of the points when he landed his first serve.

Croat Cilic only managed to win two receiving points in the first set, unable to puncture his opponent’s impenetrable serve once again at set point in the tie-breaker and paying the price when he buckled under pressure with a forehand into the net four points later.

Djokovic missed three break points in a lengthy opening game of the second set, then saw his 31-point winning streak on serve ended in the third game.

That only served to sharpen Djokovic’s focus, however, as he broke in the next game and again for a 5-2 lead before serving out to win in one hour and 36 minutes.

“Obviously I knew before the match that I had already qualified, and I knew who I’m playing, so it wasn’t easy to approach this match with 100% willingness to win it,” said Djokovic.

“But this is a unique format and both of us wanted to win from the first point. It was a close first set which probably decided the match.”

Zverev’s time to arrive on the big stage?

With the ‘big four’ of Federer, Djokovic, Rafael Nadal and Andy Murray all in their 30s, Zverev has long been identified as the young player most likely to fill the void when they retire.

The 21-year-old is yet to make a meaningful impact in the Grand Slams but, after adding eight-time major champion Ivan Lendl to his team, has reached the last four at the O2 for the first time.

Now, with Federer and possibly Djokovic standing in his way, back-to-back victories over the weekend would announce his arrival to a wider audience.

After showing mental resilience to edge past Cilic in his opening match, Zverev knew any victory against Isner would guarantee his place in the last four, despite being outclassed by Djokovic in his second.

As with the Cilic match, he delivered on the big points against the American to win a match in which there was little between the two players.

Isner out-served and knocked out

Zverev delivered an 143mph ace to wipe out a set point at 6-5 – the only break point of the first set – and went on to earn a hold which took the opener into a tie-break and ultimately proved pivotal.

Isner knew he had to win to stay in the tournament, with a two-set victory his only hope of reaching the semi-finals, but mistimed a forehand into the net on Zverev’s second set point.

The 6ft 10in American’s game is unsurprisingly built around his massive serve but he was outgunned by Zverev, who had a faster serve speed, higher first-serve percentage and more aces.

And, after Zverev took his first break point at 4-4 in the second, victory was quickly wrapped up with a hold to love and allowed him to start thinking about facing Federer.

“I’m unbelievably happy to be in the semi-finals,” Zverev said. “I’ve had such a good run over the last year.

“John was such a tough opponent and the last two matches could have gone either way for him. The first set was unbelievably tight. It’s always one or two points against him.

“I came up with some great shots in the tie-break as well and that helped because don’t want to be down with him serving for the set.”

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Roger Federer says thinking about winning his 100th singles title would only add extra pressure before his ATP Finals semi-final against young German Alexander Zverev at the O2 in London.

Federer is aiming to become only the second man – after American Jimmy Connors – to reach the landmark.

“I won’t let that get in my head,” said the 37-year-old Swiss.

The second seed faces Zverev at 14:00 GMT on Saturday before Novak Djokovic and Kevin Anderson meet at 20:00.

The winners meet in the final of the season-ending tournament at 18:00 on Sunday.

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Federer lost his opening match with a below-par performance against Kei Nishikori, before returning to form in straight-set wins over Dominic Thiem and Kevin Anderson.

“I’m happy that this is another week like this,” said Federer, who is looking to claim a record-extending seventh ATP Finals title.

“It didn’t look like it, but I was able to come back and play good tennis.

“Personally I’m still not thinking of the number 100.

“I won’t let it make me go crazy because it should be something I’m excited about and not something I should feel extra pressure.

“As long as I think Novak is in the draw anyhow, he’s playing so good again, it’s never going to be easy.”

Djokovic ‘knows what to expect’ in rematch of Wimbledon final

Djokovic is seen as the man to beat at the O2 after a remarkable run which has seen him win 34 of his past 36 matches.

The Serb was outside of the world’s top 20 in June, but rediscovered his form to win the Wimbledon and US Open titles and going on to reclaim the world number one ranking.

Djokovic, 31, earned his first Grand Slam title in more than two years by beating Anderson at Wimbledon and will be the favourite to win the rematch at the O2.

“Kevin is playing some great tennis, it’s definitely his best season and he’s continued doing that throughout this tournament,” said Djokovic.

“He’s playing well, serving well, very aggressive, so I know what to expect.”

Murray and Soares eye first final

Britain’s Jamie Murray and Brazilian partner Bruno Soares bid for their first appearance in the ATP Finals showpiece when they open semi-finals day against American pair Mike Bryan and Jack Sock at 12:00 GMT.

Murray and Soares, who have lost in the last four in each of the past two years, won their group with a 100% record of three victories.

“We’re happy to be in the semi-finals, the first goal at the start of the week is getting out of the group, no matter how you do it,” said the 32-year-old Scot.

Brazilian Soares also lost in the 2013 semi-finals alongside Austrian Alexander Peya.

“It’s about time I went one step further,” said the 36-year-old Brazilian. “I guess this is the year to go a couple more.”

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