NEW YORK — Serena Williams seemingly was cruising along to a straightforward victory in the U.S. Open’s fourth round Sunday when she took an awkward step while at the net for a volley and twisted her right ankle.

She tumbled to the ground and sat there, cross-legged, right in the middle of Arthur Ashe Stadium, for a few moments. Williams has dealt with health issues all season, including one that ended her stay at the Australian Open.

But soon enough, Williams was back on course, beating No. 22 seed Petra Martic 6-3, 6-4 to reach the quarterfinals.

Novak Djokovic, the defending champion and men’s top seed, wasn’t as fortunate with his injury.

Djokovic was forced to quit playing during the third set of his fourth-round night match against Stan Wawrinka.

“It’s never the way you want to finish the match,” said Wawrinka, who will face No. 5 seed Daniil Medvedev in the quarterfinals. “I feel sorry for Novak.”

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Djokovic, a 32-year-old Serb, had won 36 of his past 37 Grand Slam matches, and four of the last five major titles.

But he began complaining about his left shoulder during a second-round victory Wednesday, when he repeatedly got massaged by a trainer during changeovers.

Earlier, Williams had no answer for why her ankle rolled.

“I was volleying and it just went over, so that was a little frustrating,” said Williams, who will face No. 18 Wang Qiang of China next.

Never really slowed her down, though. A couple of points after falling, she got the last service break she would need with a drop volley winner that made it 3-2.

She ended this match by raising both arms after smacking an ace at 118 mph, her fastest of the afternoon, punctuating an overpowering performance that included 38 winners to Martic’s 11.

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“It’s been a rough year with injuries,” said the 37-year-old Williams, who owns six U.S. Open titles and 23 Grand Slam singles trophies in all, “so I thought, ‘Oh, no, not again!’ ”

The scene brought back memories of Williams’ Australian Open quarterfinal in January against Karolina Pliskova. Williams was way ahead – holding a match point at 5-1, 40-30 in the third set – when she turned her left ankle awkwardly.

From there, Williams was no longer, well, Williams, dropping every point she served the rest of the way.

She failed to convert four match points in all that day, ceded six games in a row and lost 7-5 in the third. That time Williams didn’t call for a trainer, which surprised many.

Didn’t make that mistake this time.

Wang, who never had played in the fourth round at a major until Sunday, advanced by surprising No. 2 seed and reigning French Open champion Ash Barty, 6-2, 6-4.

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Pliskova, the 2016 U.S. Open runner-up who was seeded No. 3, departed, too, with a 6-7 (1), 6-3, 7-5 loss over nearly 2½ hours against 16th-seeded Johanna Konta of Britain.

This is the only Grand Slam tournament where Konta has yet to reach the semifinals; she’ll now meet No. 5 Elina Svitolina of Ukraine, who defeated No. 10 Madison Keys of the United States. 7-6, 6-4.

Earlier Sunday, Roger Federer kept up his recent surge, dominating No. 15 seed David Goffin 6-2, 6-2, 6-0 in just 1 hour and 19 minutes.

That’s one fewer minute than it took him to put together a 6-2, 6-2, 6-1 win against Dan Evans in the previous round.

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