Five-time champion and world number two Federer claimed a 6-3, 6-4, 6-4 victory over his 29th-ranked German opponent.

The 34-year-old Swiss star, hoping to become the oldest champion since Ken Rosewall in 1970, will next face US 13th seed John Isner, who progressed after Czech opponent Jiri Vesely retired with a neck injury after dropping the first two sets.

Bouchard, meanwhile, was facing a battle to make her scheduled last-16 clash with Italy's Roberto Vinci on Sunday after injuring her head in a fall in the women's locker room on Friday. The 21-year-old Canadian star withdrew from the women's and mixed doubles tournament on medical advice.

"The severity of the injury was and continues to be assessed by the on-site medical personnel at the US Open," said tournament director David Brewer. "No determination about competing in the singles competition has been made at this time. Bouchard will continue with medical evaluation and treatment for the remainder of the day.

"An update on her condition will be provided later today."

Federer has now reached the fourth round in New York for the 15th year as he looks to return to the final for the first time since 2009. His win was only marred by him dropping serve for the first time since Wimbledon.

"I won the big points today, I served well when I had to, even though it was up and down for both of us," said Federer, who will take a 4-1 career lead over Isner into their fourth round clash. "He stood far back to return so I had to do a reset in my mind. Philipp's a great player.

"I have practiced with him tons so I know how tough he is."

Federer also avoided falling out of the tournament as long-time rival Rafael Nadal had done on Friday night in the Spaniard's earliest exit in New York in 10 years. Later Saturday, Britain's Andy Murray, the champion in 2012, faces 30th seed Thomaz Bellucci of Brazil, who has reached the third round for the first time.

Fifth seed Stan Wawrinka, the French Open champion, is also looking for a last-16 place when he faces Belgian world number 107 Rubens Bemmelmans, who had never won a match in New York before this year. Bemmelmans secured a place in his first third round at a major when American opponent Jack Sock retired through heat exhaustion on Thursday.

Czech sixth seed Tomas Berdych, a semi-finalist in 2012, reached the last 16 for a fourth successive year by beating Spain's Guillermo Garcia-Lopez 6-7 (2/7), 7-6 (9/7), 6-3, 6-3. He next plays 12th seed Richard Gasquet, one of four Frenchmen in the last 16, who defeated Australia's Bernard Tomic 6-4, 6-3, 6-1.

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