The world No. 4 suffered a 6-3, 6-3 defeat to Mertens in the fourth round on Wednesday, ending her hopes of reaching the quarter-finals in Beijing.
Gauff's defeat came just a day after she publicly condemned the racist messages she received following a controversial incident during her previous match against Chinese teenager Sun Xinran.
Controversial incident sparks online abuse
Gauff had defeated Sun 7-5, 6-1 in the previous round, but the match attracted attention after the American stopped play during a crucial point in the first set.
Gauff raised her hand after being distracted by flickering electronic advertising boards around the court. Sun appeared set to win the point, but the chair umpire reviewed the incident and ordered the point to be replayed.
Gauff won the replayed point and went on to complete the victory.
The decision triggered criticism online, with some users accusing the American of deliberately stopping the rally.
Gauff subsequently explained that she had followed the rules and had also apologised to Sun after the match, with the young Chinese player understanding the situation.
Gauff condemns racist messages
The criticism escalated into racist abuse directed at Gauff across social media.
The 22-year-old responded by making it clear that disagreement over a tennis incident did not justify racist attacks.
She also rejected accusations that she had cheated during the match.
The Women's Tennis Association condemned the abuse and said it was in contact with Gauff to offer its support, stressing that there is no place for racism, discrimination or abuse in tennis.
The issue highlights the continuing problem of online abuse directed at professional tennis players. The WTA has previously introduced technology designed to monitor abusive social-media activity involving players and officials.
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Gauff admits the abuse affected her
Speaking after her defeat by Mertens, Gauff acknowledged that the experience had taken a toll.
“It has been kind of draining. I'm not going to lie.”
The American said she had tried to deal with the situation by switching off her phone and moving on.
Gauff also made clear, however, that she did not want to use the abuse as an excuse for her performance against Mertens.
“I think I just didn't play well,” she said, while acknowledging that she had struggled with her movement and energy during the match.
Mertens ends Gauff's Beijing campaign
Mertens produced an impressive performance to defeat Gauff for the first time in six meetings.
The Belgian broke Gauff's serve four times and capitalised on an unusually error-prone performance from the American, who committed eight double faults and 34 unforced errors.
Mertens advanced to the quarter-finals, where she will face Iga Swiatek, who defeated American teenager Iva Jovic 6-1, 6-3.
For Gauff, the loss brings an end to her Beijing campaign after she had previously won the China Open title in 2024 and reached the semi-finals in 2025.
Gauff withdraws from Wuhan Open
Gauff's demanding week has also resulted in her withdrawal from the Wuhan Open, where she was due to defend her title.
The American said she needed to rest following what had been a particularly draining period, while world No. 1 Elena Rybakina has also withdrawn from Wuhan because of a foot injury.
Gauff's immediate priority will therefore be recovery after a difficult few days in Beijing.
Despite the disappointing exit, her response to the online abuse has once again brought attention to the wider issue of racism and discrimination in professional sport.
Gauff's China Open campaign may have ended on court, but the controversy surrounding her Beijing experience has sparked a much wider conversation about online abuse in tennis.