The US Open quarter-final between Madison Keys and Marketa Vondrousova was delayed for about eight minutes in the first game when a spectator needed medical attention in the stands at Arthur Ashe Stadium.
Keys was serving at deuce when action was halted. The chair umpire announced to the crowd that first-aid responders had been called.
Tournament referee Jake Garner went out to check on the person, who was in the lower-level seats.
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During the delay, Keys brought two fresh towels and a bottle of water to where the spectator was.
Eventually, the person was carried out and action resumed. The US Tennis Association said it was determined that the fan had a prior medical condition, and the incident was not heat-related.
The fan was taken for further observation on site and safely discharged.
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The temperature approached 35 degrees Celsius on Thursday at Flushing Meadows.
Earlier, Russian superstar Daniil Medvedev had warned a “player is gonna die” due to the heat.
Keys, the runner-up at the US Open in 2017, beat Vondrousova 6-1, 6-4 to reach the semi-finals.