Sabalenka looked bemused when an out call went against her to give Belgian Mertens the first service break and a 4-3 lead in the opening set of their quarter-final, and a check by umpire Miriam Bley confirmed the call.
The Belarusian went to look for herself and when Bley refused her call to check again, Sabalenka borrowed a phone from one of her team members and took a photo of the mark to a mixture of cheers and whistles from the crowd.
Bley then gave Sabalenka a warning for unsportsmanlike conduct but the player put the incident behind her to break back immediately and went on to win 6-4, 6-1.
Bley and Sabalenka shook hands after the match, which left the player with the impression that all was not forgiven.
"When I gave her a handshake there was a very interesting look and a very strong handshake, never had it before," Sabalenka said in her on-court interview.
Asked if she had returned the pressure in the handshake, Sabalenka said: "No, it's okay, why would I play this game with someone like her?"
Sabalenka will meet Jasmine Paolini in the semi-finals after the Italian defeated American Coco Gauff 6-4, 6-3.
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