In the fourth of a special series of interviews with Roger Federer at home in Switzerland, the 35 year old answers: What has changed more in the past 16 years – Roger Federer or the US Open?

Federer reminisces about his long hair and his tennis journey since first playing at Flushing Meadows in 2000.

His US Open journey is over for 2016, but Juan Martin del Potro leaves with a sense of belief that he belongs back among the ATP World Tour elite after a confidence-boosting run to the quarter-finals. The 2009 champion had to rely on a wild card just to make the main draw and it took a two-time Grand Slam champion, Stan Wawrinka, to end his hot streak.

Having spent much of the past seven years sidelined with wrist operations, including the past two US Opens, the 27-year-old Argentine arrived in New York City at No. 142 in the ATP Emirates Rankings. His run to the last eight included upsets of three seeded players, without the loss of a set.

Third seed Wawrinka would gain revenge for a Centre Court loss at Wimbledon. The 31-year-old Swiss stepped up to seal a 7-6(5), 4-6, 6-3, 6-2 victory on Arthur Ashes Stadium in the early hours of Thursday morning, but the result was enough to convince del Potro he was well on the comeback trail.

With Wawrinka closing on victory a boisterous New York crowd broke into chorus, urging the Argentine on. “I can lose the match, but I will never forget this,” del Potro said. “It’s bigger than winning any match.

“I’m so proud to get that from the crowd, because I have been putting in a big effort to play tennis again. They made me so happy tonight, and I don’t mind the score.”

It was del Potro’s first Grand Slam quarter-final since Wimbledon 2013 and his run catapults him back into the Top 65 in the ATP Emirates Rankings. Timing his thunderous forehand to full effect once more, del Potro admits match fitness after such extended breaks from the game is still a work in progress.

“I think my tennis is starting to respond as I want, but physically I’m still down,” he said. “I’m not in the same level as these guys. I need to just stay healthy and wait for the pre-season to get 100 per cent for next year.

“I’m already Top 100, so that’s good. Never will I ask for wild cards anymore. And everything here is positive for me. I’m so glad for that.”

Andy Murray’s hopes of passing Novak Djokovic atop the Emirates ATP Race to London suffered a setback on Wednesday with a five-set upset to Kei Nishikori on Arthur Ashe Stadium. The second seed was in the driver’s seat to claim his eighth victory from nine FedEx ATP Head2Head encounters with the Japanese World No. 6 when he surged to a two-sets-to-one lead. 

But the Japanese star, a US Open finalist in 2014, had other ideas, racing through the fourth set before closing it out for a narrow 1-6, 6-4, 4-6, 6-1, 7-5 result.

“Obviously I was in a good position up a set and a break and chances at the beginning of the fourth set, as well,” Murray said. “I could have won the match for sure.

“It happens sometimes … I have won a lot over the last few months, but couldn’t quite get it going my way today.”

Murray stood to pass Djokovic in the Emirates ATP Race for No. 1 had he gone on to win his second US Open and had the Serbian failed to reach the final.  He came into the match with a 26-1 record since finishing runner-up to Djokovic at Roland Garros. While disappointed, the Scot denied his run of success had taken its toll.

“I felt all right today. Kei has played a lot of tennis, as well, over the last few months,” Murray said. “This slam in particular – I think for all of the players, especially the ones that have done well over the summer period on the hard courts – it’s a tough one.”

He leaves New York City satisfied with his US summer campaign. A trip home to Glasgow for Davis Cup is next before he continues his quest to close the gap on the one man ranked above him.

“I have not let anyone down,” Murray said. “I tried my best. I fought as hard as I could with what I had today.

“I pushed myself as hard as I could over the last few months, and I’m very proud of how I have done. If someone had offered me the summer that I have had before Wimbledon, I probably would have signed for that.” 

Kei Nishikori turned in his latest clutch performance on Wednesday, upsetting Andy Murray in the US Open quarter-finals to reach his second career Grand Slam semi-final in New York and his first since the 2014 US Open. In a back-and-forth contest that featured 17 breaks of serve, Nishikori was ultimately the more settled player during the match’s tightest moments. He took the three-hour and 57-minute contest 1-6, 6-4, 4-6, 6-1, 7-5.

“There were many up and downs, but I tried to [stay] calm. I think that’s the most important thing I did today. Even though there were many up and downs I tried to stay tough,” Nishikori said.

The memorable performance is just the latest time “Clutch Kei” has stepped up in a decider. Heading into the match, Nishikori had been the most successful player of all-time in matches that had gone to a decider, according to the FedEx ATP Performance Zone. Nishikori owned a 96-26 record, giving him a winning percentage of 79 per cent, a higher percentage than anyone in the history of the sport.

At no point on Wednesday was Nishikori more clutch than deep in the fifth set. With Murray serving at 5-all in the decider, Nishikori attacked a second serve and, as he did 39 times in the match, he approached the net. Nishikori lunged at Murray’s passing shot and his forehand volley dropped in for an uncontested winner and the break. The sixth seed served out the set to reach the last four in Flushing Meadows.

“I saw some opportunities to come in today so I tried to be aggressive. I saw that’s what I had to do, especially against Andy. He has great defense,” Nishikori said. “I don’t know why I served and volleyed a lot today… but it was working. I think it was a great mix, serving and volleying and coming to the net.”

The previous time Nishikori reached the US Open semi-finals he went on to contest for his maiden Grand Slam final before falling to Croatian Marin Cilic. In this year’s semi-finals, Nishikori will face either Argentine Juan Martin del Potro or Swiss Stan Wawrinka, who play the late match in Arthur Ashe Stadium tonight. “Either way it’s going to be tough. I’ll try to stay calm and be ready,” Nishikori said.

Nishikori has a losing record against both of them: 2-3 against Wawrinka, and 0-4 against del Potro. But as he showed on Wednesday, the FedEx ATP Head2Head records mean little when Nishikori finds his stride in New York.

With the win, he improves to a 5-0 record against Top 10 players at the US Open. The victory also marks just the second time Nishikori has beaten the Brit during their nine FedEx ATP Head2Head meetings. In fact, before a five-set Davis Cup match earlier this season, Nishikori hadn’t even taken a set off of Murray. The Scot started their rivalry by winning the first 12 sets.

The quarter-final match in a packed Arthur Ashe Stadium began the way their past series had gone as well. The 2012 US Open champion Murray controlled the first set and looked to be well on his way to a semi-final in New York for the fourth time. The 29 year old won more than half of his return points and especially attacked Nishikori’s second serve, winning 8/10 second-serve points.

“Obviously I was in a good position up a set,” Murray said.

Read More: It’s Djokovic’s Decade In New York

But the sixth seed Nishikori bounced back in the second set, mixing in drop shots and more slice and refusing to simply rally against the No. 2 player in the Emirates ATP Rankings. The Shimane, Japan, native said he especially benefited from a brief chat with his coach Michael Chang, a former World No. 1 and a finalist at the 1996 US Open. The two talked as officials covered Arthur Ashe Stadium with its new roof at 3-all.

“After the rain delay I think I improved a little bit with my coach, and I tried to change my tennis and it started working better. I started to get my rhythm back,” Nishikori said.

In the third set, neither player could take control. The set featured five breaks of serve, and the most crucial came at 4-all on Nishikori’s racquet. The 26 year old was serving 30/40, 4-4, when he tried another drop shot but floated it wide to give Murray the break. After a hold to start the fourth, it looked like Murray would, at last, seize control of the quarter-final contest and continue his stellar stretch into the semi-finals. Since losing in the Roland Garros final, the Brit had gone 26-1 and reached seven consecutive finals.

Read More: Murray Strolls Into QFs

But Nishikori would not go away. At 1-0, during a break-point opportunity for Murray, a loud noise in Arthur Ashe Stadium caused a let. Murray looked flustered after the disturbance, and Nishikori took advantage. The 2014 US Open finalist reeled off six straight games to even the match and force a deciding fifth set.

In the decider, “Clutch Kei” showed how he has achieved a higher winning percentage in matches that go to a decider than anyone in tennis history. Nishikori stayed calm. Even after he broke Murray for a 6-5 lead in the fifth, Nishikori didn’t jump or shout with excitement; he simply walked to his chair and prepared to hold and reach the semi-finals at the US Open.

Top seeds Nicolas Mahut and Pierre-Hugues Herbert are a match away from playing for a second consecutive US Open doubles title. The Frenchman reached the semi-finals in Flushing Meadows on Wednesday with a 6-3, 7-6(4) win against Swede Robert Lindstedt and Pakistani Aisam-Ul-Haq Qureshi.

Mahut/Herbert hit eight aces and never faced break point in the one-hour and 24-minute match. They’ll face familiar opponents in the semi-finals: Jamie Murray and Bruno Soares. The fourth seeds outlasted Aussie Chris Guccione and Brazilian Andre Sa 7-6(9), 2-6, 6-3 in two hours and 23 minutes. Murray/Soares erased seven of nine break points faced and won exactly one more point than their opponents.

Read: Murray/Soares Qualify For London

“Happy to get through today,” Murray said. “It was a stressful match. We came back strong in the third set.”

Murray/Soares played Mahut/Herbert in the Monte-Carlo Rolex Masters final on clay in April. The Frenchmen won the ATP World Tour Masters 1000 final 4-6, 6-0, 10-6. Murray, playing with Aussie John Peers, also faced the duo in last year’s US Open doubles final, which Mahut/Herbert won 6-4, 6-4.

“Tomorrow we will be more relaxed, ready to give our all,” Murray said. “We have nothing to lose and everything to gain. Game on.”