* Del Potro recovers from slow start
* Tomic also through in Sydney
* Errani joins seed exodus (Adds quotes, details)
By Ian Ransom
MELBOURNE, Jan 8 (Reuters) - World number five Juan Martin del Potro, the man tipped to challenge for grand slam titles this season, began his year with a scrappy win over Frenchman Nicolas Mahut to advance to the quarter-finals 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 U.S. 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.
Other seeds struggled, with No.3 Andreas Seppi from Italy 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.
Defending champion Bernard Tomic advanced with a hard-fought 6-3 4-6 6-4 win over Slovenian qualifier Blaz Kavcic, continuing his habit of raising his game for home fans at the start of the year.
The rangy 21-year-old, ranked 52nd in the world, has disappointed some in Australia for failing to live up to sky-high expectations and has been drawn into controversy by his coach and father John Tomic.
Tomic senior has been barred from the Australian Open after a court in Spain convicted him of assaulting his son's former hitting partner last year.
"The majority of the crowd like me in Australia," the 21-year-old Tomic told reporters after setting up a quarter-final with Ukraine's Alexandr Dolgopolov.
"At the end of the day, if you win, it's going to put a zip to everyone."
The women's draw, which has already seen top seed Agnieszka Radwanska and former world number one Caroline Wozniacki sent packing, lost third seed Sara Errani, last year's surprise French Open semi-finalist, who was upset 7-6 (7-2) 6-3 by Bulgaria's Tsvetana Pironkova in the quarter-finals.
The Italian world number seven took a medical time-out in the second set to have treatment on her lower back, raising concerns over her fitness ahead of the year's first grand slam.
Former Wimbledon semi-finalist Pironkova will play second seed and former Wimbledon champion Petra Kvitova, who wore down Lucie Safarova 7-6 6-2, for a place in the final.
The invitational Kooyong Classic kicked off in Melbourne's leafy eastern suburbs on Wednesday, with a smattering of men's top 10 players warming up at the eight-man exhibition, which offers the same surface used at Melbourne Park.
World number seven Tomas Berdych roared back from a 5-2 deficit in the first set to overhaul Spaniard Fernando Verdasco 7-5 6-2 in his first hit-out of 2014 in brilliant sunshine.
Japanese number one Kei Nishikori also enjoyed an opening win over Bulgaria's Grigor Dimitrov 7-6 (7-5) 6-4.
World number nine Richard Gasquet of France nearly suffered an embarrassing defeat by teenager Jordan Thompson, and had to save two match points before fending off the 321st-ranked Australian 7-5 3-6 7-6 (7-4).
Sam Stosur's stuttering revival after a dismal Hopman Cup continued with a second straight win at the Hobart International in front of home fans.
The former U.S. Open champion had to save two match points against 54th-ranked Frenchwoman Kristina Mladenovic as she laboured through a 6-4 2-6 7-6 (9-7) match blighted by an astonishing 22 double-faults from both players.
World number three David Ferrer, bidding for his fourth straight Auckland Open crown, began his 10th appearance in the event with a 6-7 (7-5) 6-3 6-1 win over American Donald Young. (Editing by John O'Brien and Ken Ferris)