U.S. Open champion Naomi Osaka extended her winning streak to 13 matches on Wednesday but only after riding out a stiff challenge from Katie Boulter to beat the Briton 3-6 6-3 6-1 and reach the quarter-finals of the Gippsland Trophy.

Boulter beat American Coco Gauff in the second round of the Australian Open warm-up and another upset looked on the cards when she matched her Japanese opponent's power-hitting to take the opening set on Margaret Court Arena.

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The second seeded Osaka, who won the Australian Open in 2019, raced to a 3-0 lead in the second set but the 24-year-old Briton would not fold and had break points to put the stanza back on serve.

Osaka upped her return game considerably to wrest back the momentum and level up the match before running away with the third set and sealing a last eight tie against Irina-Camelia Begu who saw off another Briton, Johanna Konta, 4-6, 7-6 (10), 7-6 (4).

"After I lost the first set I thought I just want to fight as hard as I can," said Osaka, who is playing her first WTA tournament since securing her second title at Flushing Meadows in September.

"She was playing really amazing and I thought 'if I lose okay, but I have to give it my best shot'. I like to play long matches ... and I played a short match yesterday so maybe I made myself play a long match today."

Top seed and world number two Simona Halep is in action in her third round match against German Laura Siegemund on the same court later on Wednesday.

Meanwhile at the Yarra Valley Classic the top two seeds Ash Barty and Sofia Kenin overcame steep challenges on Wednesday to reach the quarter-finals and keep their Australian Open preparations on course.

World number one Barty, who won 6-0 4-6 6-3, showed no signs of fatigue from having played a late match on Tuesday and started in top gear against Marie Bouzkova at the Margaret Court Arena before her Czech opponent staged a fightback.

Ash Barty

Image credit: Getty Images

Australian Open champion Kenin, seeded second, appeared headed for an exit against fellow American Jessica Pegula when she fell 3-0 back after losing the first set but the feisty 22-year-old found her range and rallied for a 5-7 7-5 6-2 win.

Barty, Australia's best hope for a Grand Slam champion this month, punished Bouzkova's weak serve to hand her a bagel in the opening set but the world number 52 showed she had the stomach for a fight by levelling the match with a break in the second.

Bouzkova and Barty traded two breaks of serve at the start of the decider before the top seed got the crucial break in the eighth game then sealing the encounter on her first match point with a booming serve.

"Coming off a night match, I had to get used to going to bed quite late again," Barty, who hit 42 winners to her opponent's 17, said in her on-court interview.

I'm an early riser, so I didn't get a hell of a lot of sleep last night, but it's fantastic to be back out here.

"It was a really solid match. I felt like I pressed a little bit in the middle and lost my execution, but I'm glad to have won and get another opportunity to play tomorrow."

The 2019 French Open champion will next face American Shelby Rogers, who ousted Croatia's Petra Martic 7-6(1) 6-3.

Kenin, ranked fourth in the world, struggled with her unforced errors in the first half of the match but once she managed to land the ball closer to the lines, a bewildered Pegula faded away.

The Russian-born Kenin hit 14 winners more than her opponent in the match while committing 10 extra unforced errors and converted her third match point with a wide serve to set up a last eight clash against either Spaniard Garbine Muguruza or Anastasia Pavlyuchenkova of Russia.

Serena Williams sounded a warning for her fellow Australian Open contenders with a sleek 6-1 6-4 victory against Bulgarian Tsvetana Pironkova to reach the quarter-finals of the Yarra Valley Classic in Melbourne.

Williams, a 23-times Grand Slam winner and a seven-time Australian Open champion, moved well, looked composed and mixed power with poise against Pironkova, who had won a set off her during last year's U.S. Open quarter-finals.

The 39-year-old American did not allow her a foothold in Wednesday's match and hit 25 winners against the seven managed by Pironkova, who like Williams returned to professional tennis after becoming a mother.

"It's definitely nice to get another win," Williams, who will next face Czech Karolina Pliskova or Danielle Collins for a semi-final spot, said during her on-court interview.

"She's clearly a great player, so it wasn't easy, but it was good to come through."

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