DOHA: An irritated world No. 1 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 had knocked out Maria Sharapova in a previous round — but 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.

She now plays a former Australian Open champion, Angelique Kerber, in the last eight on Friday.

The German came from a set down to beat Johanna Konta 1-6, 6-1, 6-3.

The woman Wozniacki beat in Melbourne, Simona Halep, is also through beating Anastasija Sevastova 6-4, 6-3, keeping alive the possibility of a replay of the Australian Open final.

Halep shrugged off injury concerns coming into the match about an ankle injury which had kept her out of action since Melbourne.

She seemed relatively untroubled breaking her opponent's serve late in each set to advance to the last eight.

"I'm really glad that I could finish it in two sets and play like I played," said Halep.

The world No. 2 will play 18-year-old American qualifier Catherine Bellis in the last eight, who beat defending champion and world No. 5 Karolina Pliskova 7-6 (7/4), 6-3.

It is the first time the teenager has beaten anyone ranked in the top five.

"I think it's definitely a milestone. It's great," she said.

Garbine Muguruza, world No. 4, breezed through beating Sorana Cirstea 6-0, 6-4 and will play France's Caroline Garcia in the next round.

And a resurgent Petra Kvitova beat world No. 3 Elina Svitolina 6-4, 7-5 to set up a quarter-final clash with another top-ten player, Germany's Julia Goerges.