Juan Martin del Potro returns to Nicolas Mahut during their match at the Sydney International on Wednesday. Photo: CFP

World No.5 Juan Martin del ­Potro ­began his 2014 season with a win, ­albeit a scrappy one over ­Frenchman ­Nicolas ­Mahut to ­advance to the quarter­finals of the Sydney International on Wednesday.

The Sydney warm-up to the ­Australian Open has proved a graveyard for the seeds this week but the 25-year-old former US Open champion avoided an ­early flight to Melbourne by coming back to win 1-6, 6-3, 6-4.

Rated a dark horse to win the ­title at ­Melbourne Park and add to his sole Grand Slam triumph at Flushing Meadows in 2009, the top-seeded Argentine will next take on Czech veteran Radek Stepanek for a place in the last four.

Del Potro said ­Stepanek, who ­defeated ­Spaniard Albert Ramos 6-2, 7-6 (7/5), will be a tough opponent early in the season.

"They won the ­Davis Cup [in ­November], so he [­Stepanek] must feel confident playing on this surface. He has experience. We play many times. ­Every match was close," Del ­Potro said.

Other seeds ­struggled, with third-seeded Italian Andreas ­Seppi bundled out by Australia's Marinko Matesovic 6-3, 6-4 and sixth seed ­Julien Benneteau of France dumped in straight sets by Ukraine's Sergiy Stakhovsky.

Bernard Tomic kept his ­Sydney title defense on track with a three-set ­victory over Slovenian ­qualifier Blaz Kavcic (6-3, 4-6, 6-4).

"It was Kavcic's third or fourth match. He qualified and won against Jarkko Nieminen, who was a winner here in the past," Tomic said.

"I knew his scores were solid and it was going to be a tough match. Would have loved to finish it off in two sets, but I'm happy I won in the end."

In the women's draw, former Wimbledon champion Petra Kvitova won the battle of the Czech left-handers to reach the semifinals of the Sydney event.

The second seed overcame world No.27 Lucie Safarova 7-6 (7/4), 6-2 for her 300th career win to advance to the last four, where she will face Bulgarian Tsvetana Pironkova.

American Bethanie Mattek-Sands, who upset top seed Agnieszka Radwanska, retired with a back injury while trailing fellow American Madison Keys 3-2 in their night quarterfinal.

Keys will face German fifth seed Angelique Kerber, who ousted Spaniard Carla Suarez Navarro 6-2, 6-4.

Pironkova, a semifinalist at Wimbledon in 2010, upset third seed Sara Errani of Italy 7-6 (7/2), 6-3.

Mattek-Sands said she withdrew to nurse her body through to next week's Australian Open and beyond.

"For me, I want to take care of my body. It's not even just about Australian Open, it's about the rest of my year," she said.

Agencies - Global Times