New York - Roger Federer defeated Philipp Kohlschreiber for the 10th time to reach the US Open last 16 on Saturday as Canadian golden girl Eugenie Bouchard suffered a potentially tournament-ending head injury.
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 Garca-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.
In the women's event, two-time Wimbledon winner Petra Kvitova, the Czech fifth seed, eased past Anna Karolna Schmiedlova of Slovakia 6-2, 6-1.
She next faces British qualifier Johanna Konta, the world number 97 who beat German 18th seed Andrea Petkovic 7-6 (7/2), 6-3.
Konta will be playing in her first Grand Slam fourth round.
"I'm just happy to win it," Konta said. "It was definitely tough."
Sydney-born Konta joined Laura Robson from 2012 as the only British women in the fourth round on the New York hardcourts since Jo Durie in 1991.
No British woman has reached the US Open final since Virginia Wade defeated Billie Jean King in the 1968 final.
Also advancing was Australia's 22nd-seeded Samantha Stosur, who ousted Italian 16th seed Sara Errani 7-5, 2-6, 6-1.
Later Saturday, second-seeded Romanian Simona Halep faces America's Shelby Rogers, a qualifier ranked at 154 in the world.
Halep has reached at least the quarter-finals of all the Slams with the exception of the US Open, where her best run was a last-16 spot in 2013.
Former world number one and two-time US Open runner-up Victoria Azarenka meets German 11th seed Angelique Kerber.
Azarenka has a 4-0 career lead over Kerber.
Latest US Open results on Saturday, the sixth day of the 2015 tournament at Flushing Meadows (x denotes seeded player):
Tomas Berdych (CZE x6) bt Guillermo Garca-Lpez (ESP x31) 6-7 (2/7), 7-6 (9/7), 6-3, 6-3
Richard Gasquet (FRA x12) bt Bernard Tomic (AUS x24) 6-4, 6-3, 6-1
John Isner (USA x13) bt Jir Vesel (CZE) 6-3, 6-4 - retired
Roger Federer (SUI x2) bt Philipp Kohlschreiber (GER x29) 6-3, 6-4, 6-4
Petra Kvitova (CZE x5) bt Anna Karolna Schmiedlov (SVK x32) 6-2, 6-1
Johanna Konta (GBR) bt Andrea Petkovic (GER x18) 7-6 (7/2), 6-3
Samantha Stosur (AUS x22) bt Sara Errani (ITA x16) 7-5, 2-6, 6-1