Bernard Tomic withdrew from his match at the US Open because of a left hip injury on Saturday (NZT), sending fourth-seeded David Ferrer into the third round.
Tomic had surgery on both hips in January. He said the discomfort he felt starting in the first round wasn't in the same spot, but he didn't want to risk aggravating it, especially against somebody like Ferrer who runs his opponents around the court.
Tomic planned to visit a doctor to figure out what's wrong. He said he doesn't "want to muck around with that area."
The 21-year-old Tomic has never been past the second round at the US Open.
Frenchman Gael Monfils also progressed to the next round, but he did so in a far from clinical display.
The 20th-seeded Monfils wound up with 10 double-faults in all, but that did not prevent him from beating Alejandro Gonzalez of Colombia 7-5, 6-3, 6-2 to reach the third round at Flushing Meadows for the first time since a quarterfinal run in 2010.
He will next face fellow Frenchman Richard Gasquet.
Meanwhile, on the women's side, Peng Shuai beat another seeded player to reach her second straight Grand Slam round of 16.
Two days after upsetting fourth-seeded Agnieszka Radwanska, Peng defeated 28th-seeded Roberta Vinci 6-4, 6-3 on Saturday. The Chinese veteran had gone two years without making it past the second round at a major tournament before her Wimbledon run this summer. But she didn't meet any seeded players in the first three rounds there before losing to eventual champion Petra Kvitova in straight sets.
This time, Peng, ranked 39th, will face 14th-seeded Lucie Safarova or 22nd-seeded Alize Cornet to try to reach her first Grand Slam quarterfinal.
Peng said she'll need "more courage" in that match.
She'll have the momentum of two straight sharp performances. With third-ranked Li Na sidelined by a knee injury, Peng's success is giving the fans in China something to cheer.
Ninth-seeded Jelena Jankovic also advanced by routing Johanna Larsson 6-1, 6-0. The 96th-ranked Larsson had upset Sloane Stephens in the second round.
And Sara Errani ended Venus Williams' run in a comeback victory. Two points from defeat, the diminutive Italian rallied to force a third-set tiebreaker and outlasted Williams in a match that was both lopsided and tight.
Errani won 6-0, 0-6, 7-6 (5) after Williams had a chance to serve out the match in the final set.
Williams twice came back from down a break in the third but was undone by too many unforced errors - she made 52 in all. Trying to close out the match at 5-3, the 19th-seeded Williams had one of her eight double-faults.
The 13th-seeded Errani next faces qualifier Mirjana Lucic-Baroni, who stunned second-ranked Simona Halep 7-6 (6), 6-2.
The 32-year-old Lucic-Baroni hadn't made a Grand Slam round of 16 since 1999, waylaid by injuries and financial struggles.
Men's singles - second round
David Ferrer (4), Spain, def. Bernard Tomic, Australia, walkover.
Grigor Dimitrov (7), Bulgaria, def. Dudi Sela, Israel, 6-1, 6-2, 6-2.
Dominic Thiem, Austria, def. Ernests Gulbis (11), Latvia, 4-6, 3-6, 6-4, 6-3, 6-3.
Richard Gasquet (12), France, def. Paolo Lorenzi, Italy, 7-6 (4), 6-3, 6-3.
Marin Cilic (14), Croatia, def. Illya Marchenko, Ukraine, 7-6 (2), 6-2, 6-4.
Roberto Bautista Agut (17), Spain, def. Tim Smyczek, United States, 6-3, 6-2, 6-1.
Kevin Anderson (18), South Africa, def. Jerzy Janowicz, Poland, 6-7 (6), 6-2, 6-1, 6-3.
Feliciano Lopez (19), Spain, def. Tatsuma Ito, Japan, 6-4, 3-6, 6-4, 7-6 (4).
Gael Monfils (20), France, def. Alejandro Gonzalez, Colombia, 7-5, 6-3, 6-2.
Gilles Simon (26), France, def. Federico Delbonis, Argentina, 6-4, 3-6, 7-5, 6-1.
David Goffin, Belgium, def. Joao Sousa (32), Portugal, 6-4, 6-2, 6-0.
Women's singles - third round
Mirjana Lucic-Baroni, Croatia, def. Simona Halep (2), Romania, 7-6 (6), 6-2.
Belinda Bencic, Switzerland, def. Angelique Kerber (6), Germany, 6-1, 7-5.
Jelena Jankovic (9), Serbia, def. Johanna Larsson, Sweden, 6-1, 6-0.
Sara Errani (13), Italy, def. Venus Williams (19), United States, 6-0, 0-6, 7-6 (5).
Lucie Safarova (14), Czech Republic, def. Alize Cornet (22), France, 6-3, 6-7 (3), 6-4.
Peng Shuai, China, def. Roberta Vinci (28), Italy, 6-4, 6-3.