China's Wu Yibing collapsed and was helped off the court on Monday after retiring from his opening match against Japan's Yosuke Watanuki at the ATP and WTA DC Open.
It was the second notable health incident in a match within a month for Wu, who needed a medical timeout after falling ill at the end of the first set in his opening-match Wimbledon loss to American Frances Tiafoe.
On the Washington hardcourts, Wu was leading 4-1 in the first set when he made a wobbly step, went to take a seat and collapsed forward over the back of a folding chair.
Wu was helped to his feet and eased down onto the court with an umbrella quickly brought over to provide shade. He was later assisted in walking off the court.
A statement from the tournament said Wu retired due to illness.
The 23-year-old from Hangzhou also played last week in Atlanta, beating France's Corentin Moutet in his first match before losing to eventual champion Taylor Fritz, the world No 9 from the United States, in the second round.
World number 90 Wu won his first ATP title in February at Dallas, becoming the first Chinese player to capture an ATP crown and China's highest-ranked player ever, reaching 54th in the world in May.