Roger Federer defeated Philipp Kohlschreiber in straight sets to reach the quarter-finals of the US Open but there were more concerns about the 36-year-old's fitness.

Federer, who came into the tournament nursing a suspect back, took an off-court medical time-out at the end of the second set but did not appear hampered as he claimed a 6-4, 6-2, 7-5 victory.

The Wimbledon champion is through to a 12th quarter-final at Flushing Meadows where he will face Juan Martin del Potro in a rematch of the 2009 final following the Argentinian's epic comeback to beat Dominic Thiem.

Explaining the reason for his departure from the court, Federer said: "I just needed a bit of a rub on my back, my bottom and I didn't want to do it on court. I wanted to get it done as soon as possible so Philipp wouldn't have to wait.

"It was a great match for me against Philipp, he can be very tough but I protected my serve very well and that helped me tonight."

The third seed is now looking forward to a first meeting in New York against Del Potro since 2009, when the Argentinian won his only grand slam title.

Federer said: "It's great to see him back on tour. He's had a rough ride for a lot of the years. To see him back, happy and winning and beating the best players in the world is wonderful for our sport.

"To play him here after our epic final in 2009 is great and I think we're both looking forward to it, or at least I am."

Pablo Carreno Busta, meanwhile, defeated Diego Schwartzman to reach his first grand slam singles semi-final.

The 12th-seeded Spaniard, the highest-ranked player left in the bottom half, proved too solid for his opponent, winning 6-4, 6-4, 6-2.

Having reached his first slam quarter-final at the French Open earlier this summer, Carreno Busta will now take on Sam Querrey or Kevin Anderson for a place in the final.

The 26-year-old dropped his racket and pumped his fists in celebration, and he said: "It's incredible. It's something that I always dreamed of but never think I can arrive here.

"It's very exciting to be in the semi-finals. It was a very good match today. I tried to be focused all the time, Diego is a fighter. You need to be aggressive all the time and try to dominate the points."

Jamie Murray and Bruno Soares' defence of their US Open title came to an end, meanwhile, with a quarter-final defeat by Jean-Julien Rojer and Horia Tecau at the US Open.

The Scottish-Brazilian pair had eased into the last eight without dropping a set but had a nightmare start and could not recover, losing 6-1, 6-2.

Murray and Soares swiftly found themselves 5-0 down to Dutchman Rojer and Romania's Tecau on Grandstand.

They at least prevented a love set and made a better start to the second but Soares' serve was broken in the fifth game and that spelled the beginning of the end.

Murray, who has been struggling with knee pain, and Soares failed to make much of an impression on their opponents' serve, creating only two break points and failing to take either.

The defeat ends a disappointing season in the slams for Murray and Soares, who won the Australian Open and US Open titles last year but have failed to make it past the quarter-finals in any of the four tournaments in 2017.