Stefanos Tsisipas nearly did something that cost Djokovic the US Open a few years ago and that's hitting somebody on the court with a tennis ball as he nearly avoids disqualification.
A few years ago Djokovic was disqualified from the US Open after hitting a court umpire with a tennis ball and Tsitsipas nearly did the same. He was up 2-0 on Lehecka when he slammed a ball into the wall out of frustration and nearly hit a ball kid. He avoided that by mere seconds and centimetres with John Courier, who commentated on the match, explaining what would have happened if actually hit the ball kid.
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People have been hit by balls on the court before but players get excused if it happens during play. When it happens away from a play, and when the action was clearly reckless then players get punished. Considering the score it would be catastrophic. He did something similar at Wimbledon and nearly hit a spectator.
Check it out below.