According to Serena Williams' coach, the tennis star was in "acceptable" pain when she defeated the No. 22 seeded Petra Martic in the fourth round of the U.S. Open to reach the quarterfinals.

Tt was during the second set of the match that Williams rolled her right ankle. As she took a step at the net, Williams fell to the ground and could be seen clutching her ankle for a few moments. She was immediately treated by a trainer who taped her lower right leg before resuming.

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"She started going for even more, I think. She felt like maybe she couldn't move that well anymore," remarked her opponent Martic. "Her shots were really precise. Serve was unbelievable. And when she hits those backhands and those first serves that well, it's not easy to play against her."

Ultimately, Williams ended the match by hitting an ace at 118 mph.

"I have to show up or else I'm going to go home," Serena noted post-match. "I feel like in this tournament, I guess, I have definitely turned a different zone. I'm not sure if I can articulate what zone that is. But it's not about kind, it's not to myself, it's not about anything in particular. It's definitely something different, though."

Next up, Serena Williams will face off No. 18 Qiang Wang during Tuesday's quarterfinals.