An irritated Caroline Wozniacki progressed to the Qatar Open quarter-finals on Thursday in a feisty match where she complained and apparently mocked her opponent Monica Niculescu's grunting.
The recently crowned Australian Open champion won 7-5, 6-1 against the Romanian - who knocked out Maria Sharapova in a previous round.
However, the world number one was visibly agitated in the ninth game of the first set, complaining her opponent was making noises as the Dane was about to hit the ball.
Wozniacki was so upset about the sounds coming from the other side of the net that she complained to French umpire Emmanuel Joseph.
"It isn't in the rule book that you are not allowed to grunt when the opposing player hits?" courtside microphones caught her asking Joseph.
The umpire said he thought everything was "regular", but Wozniacki added: "It's the only way she can win."
She then mimicked the grunts.
Afterwards she complained that her opponent was "a player that tries to get into your head".
She also told Danish TV that her opponent had used "unfair methods".
"(I) just tried to let him know to pay attention to her grunting because she'll hit the ball and two seconds later when the ball is on my side and I'm right about to hit, she'll start grunting and make a noise," the Dane told reporters.
"And she will change the grunt according to what she feels like."
Asked if she thought her opponent was grunting deliberately, Wozniacki responded: "I think that she didn't do it in the second set, so..."
The spat overshadowed a relatively routine victory for the 27-year-old, which ensured she retained her number one ranking after Simona Halep also advanced to the final eight.
By reaching the quarter-finals, Halep now has a chance of reclaiming the world's top spot if she goes one round further than the Dane.
The tournament could also see a rematch of the Australian Open final, as the pair are seeded one and two in Doha.
Halep beat Anastasija Sevastova 6-4, 6-3, hitting 29 winners on her way to victory.
She also shrugged off injury concerns coming into the match about an ankle injury which had kept her out of action since Melbourne.