Coco Gauff lost to a player she had beaten in all five of their previous matchups before saying she felt drained over what she previously described as an "insane" amount of racist comments on social media following a controversial point in the preceding round.
The 19th-ranked Elise Mertens beat the fourth-ranked Gauff 6-3, 6-3 on Wednesday to reach the China Open quarterfinals.
"It has been kind of draining, I'm not going to lie," Gauff told reporters when asked if the abusive comments had affected her. "I just turn off my phone and move on. There's nothing you can do."
Sun appeared about to win the point when Gauff stopped playing. The chair umpire ordered the point to be replayed.
"Hey everyone, you can dislike me or disagree about what happened without being racist," Gauff, who is Black, said in a reply on X. "The amount of racist comments/dms I've received in the last few hours is insane. Lastly, I'm not a cheater or a thief."
Against Mertens, Gauff dropped her serve four times, hit eight double faults and committed 34 unforced errors.
"I think today I was just a bit flat," Gauff said. "I just didn't play well. I think I wasn't moving well. There were a lot of things I didn't do well."
Mertens reached her first quarterfinal at a WTA 1000 event -- the level just below the Grand Slams -- in five years.
"She's an incredible player, of course. I knew it was going to be a very tough match," Mertens said. "But I had motivation. My coach/boyfriend, it's his birthday tomorrow, so this win is dedicated to him and all the work we do."
Mertens improved to 3-3 against top-10 opponents this year. She also beat defending champion Jasmine Paolini at the Italian Open and current No. 1 Elena Rybakina in the third round of Wimbledon, where she reached the quarterfinal.
Earlier, Iga Swiatek broke Iva Jovic's serve twice in the last four games to beat the 18-year-old American 6-1, 6-3.
Swiatek, a six-time Grand Slam champion who has struggled at times this year, will next play Mertens.
The Associated Press and PA contributed to this report.