Tsvetana Pironkova wins Sydney
Pironkova, who was ranked outside the top 100 despite reaching the Wimbledon semifinals in 2010, beat three top 10 players in Sydney, 2012 French Open finalist Sara Errani, 2011 Wimbledon champion Petra Kvitova and Kerber.
The 2009 U.S. Open champion had 6-4, 6-2 semifinal win Friday over fourth-seeded Dmitry Tursunov. Tomic had a 6-7 (4), 7-5, 6-3 win over Sergiy Stakhovsky, who last year beat Roger Federer in the second round at Wimbledon.
Tomic's win came only hours after he was drawn to face No. 1-ranked Rafael Nadal in the first round next week at the Australian Open.
The 21-year-old Australian raced to a 4-1 lead in the first set but Stakhovsky broke back and rallied to win it in a tiebreaker.
Tomic broke the No. 101-ranked Stakhovsky in the 11th game of the second set and leveled the match on his fourth set point.
He got the decisive break in the seventh game of the deciding set, and then broke his Ukraine rival's serve again in the ninth to win on his third match point.
"I was very happy to get out of this match. He should have won," Tomic said, adding that he'd have to improve vastly to have any chance of winning the final against del Potro, who beat him in straight sets in their only previous meeting.
Del Potro, who is also in Nadal's quarter of the draw at Melbourne Park, said he was only concerned about the Sydney final for now.
"I'm trying to be professional. This tournament is not over yet for me," he said. "My team already knows they're not allowed to talk about Melbourne yet. My friends, too."
Unseeded Lu Yen-hsun of Taiwan stopped David Ferrer's bid for a fifth singles title at the Heineken Open in Auckland, New Zealand, with a 6-4, 7-6 (4) win in his first-ever semifinal at ATP level.
Ferrer won the Auckland title in 2007 and in each of the last three years but dropped serve twice and made 20 unforced errors in the first set.
"I know he's a tough guy and I tried to play my best tennis and fortunately I did it," Lu said. "He's not playing bad, I just played unbelievable."
Ferrer is seeded No. 3 for the Australian Open.
In a preview of their first-round match at the Australian Open, Klara Zakopalova of the Czech Republic beat 2011 U.S. Open champion Sam Stosur 6-3, 6-2 in the semifinals of the WTA event in Hobart.
Zakopalova will next play Garbine Muguruza, who beat Estrella Cabeza Candela 6-0, 6-1 in a semifinal between two Spanish qualifiers.