The changeover was a scary scene in Washington, DC.
Yibing Wu collapsed during a changeover of his opening-round Mubadala Citi DC Open match.
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Wu held a 4-1 lead over Japan's Yosuke Watanuki when he walked slowly and unsteadily to his court side seat. When he arrived, Wu collapsed over the folding chair and fell.
Officials immediately rushed to his aid.
Initially, Wu rose to his feet, but had to sit back down again.
The 23-year-old Chinese was eventually helped off the court and was forced to retire.
Conditions in Washington, DC can be among the most sweltering on Tour. Though today was not the type of extreme heat that players have experienced in the past.
The tournament cited "illness" as the official reason for Wu's retirement.
It was the second time Wu has suffered a frightening collapse on court this month.
In his Wimbledon first-round loss to Frances Tiafoe, Wu had to stop play with an accelerated heart rate. He did return to complete the match bowing in three sets to Tiafoe.
"I mean, no, I had no idea what was going on. I was just, I was like, man, this is wild," Tiafoe said on court afterward. "Then he, yeah, he told me that his heart was skipping a beat. Having problems with his heart and stuff. Felt like he was collapsing in there.
"He's like, I'm gonna try and play."
These recent health scares come during a breakout season for Wu, who defeated Taylor Fritz and John Isner (in a triple tiebreaker thriller final) in succession to capture the Dallas Open and make history as the first Chinese man to win an ATP title in the Open Era.