The American won 7-5, 6-1 to reach the fourth round, but was targeted after stopping play during a crucial point in the first set, raising her hand to complain about a flickering advertising board just as Sun was about to put away a winner.
After a video review, the umpire ordered the point to be replayed. Gauff won the replayed point and held serve for 5-5 before cruising to victory against the Chinese 16-year-old.
Gauff, 22, said she had received a barrage of abusive messages on social media after the match.
"You can dislike me or disagree about what happened without being racist," Gauff, who is Black, said on X early on Tuesday in reply to a post containing a racial slur.
"The amount of racist comments/direct messages I've received in the last few hours is insane.
"Lastly, I'm not a cheater or a thief."
Reuters has contacted the WTA, the governing body of women's tennis, and tournament organisers in Beijing for comment.
The WTA has made efforts to tackle the issue of online abuse by introducing an AI-powered system which monitors thousands of players and officials on social media.
A study by the WTA and World Tennis showed players were subjected to over 12,000 abusive posts and messages on social media in 2025, up from 8,000 posts a year earlier.
Gauff also explained that she was following the rules when she challenged the point. Players are allowed to request video reviews of select judgment calls, including claims a distraction may have interfered with play.
The American said she had spoken to Sun about the point at the end of the match.
"Xinran's an incredible talent and player. I apologised for the situation at the end of the match, and she completely understood," she wrote.
"I didn't like to see her so emotional, as I've been there and I really understand what it's like, but this is the beginning of an incredible career. It was an honour to share the court in front of her home crowd."
