SYDNEY (AP) -- Top-ranked Angelique Kerber lost her opening match at the Sydney International on Tuesday, upset 7-6 (5), 6-2 by Daria Kasatkina in a setback ahead of her Australian Open title defense next week.
Kerber, who won last year's Australian and U.S. Open for her first major titles, has opened the new season with one win in two tournaments in Brisbane and Sydney. The Australian Open starts Monday in Melbourne.
Eugenie Bouchard continued her resurgent run, meanwhile, with a 6-4, 6-3 win over WTA Finals champion Dominika Cibulkova to reach the quarterfinals in Sydney.
Bouchard improved her career record to 4-1 against Cibulkova, who was ranked No. 5 and was voted the WTA's comeback player of the year in 2016 after her title win at the season-ending championship.
"Any time you play one of the best players in the world it's like a standard of where you're at," said Bouchard, who had a wild-card entry in Sydney. "Every time I have played a match with Domi, we have had the toughest battles.
"I have won, I guess, more often than not, but everyone is a really tough battle."
Bouchard had an early break in the second set, but dropped serve and allowed Cibulkova, a finalist at the 2014 Australian Open, a way back into the match.
But the Canadian star broke again quickly and held onto the advantage.
Bouchard had a breakout season in 2014, reaching the semifinals at the Australian Open and French Open before making the final at Wimbledon. But she has only gone past the fourth round at a major once since then and slipped to No. 46 at the end of last year.
After winning back-to-back matches at a tournament for the first time since August in Cincinnati, she will play a quarterfinal against Anastasia Pavlyuchenkova, who beat defending champion Svetlana Kuznetsova 7-5, 6-3.
"I have taken a couple of solid steps this week, but I'm far from where I want to be," Bouchard said. "Even though I won, to me, I could do a lot of things better."
Barbora Strycova had a 6-2, 6-3 win over No. 9-seeded Roberta Vinci.
At the ASB Classic in Auckland, New Zealand, fifth-seeded Spaniard Albert Ramos-Vinola lost 6-1, 7-5 in the first round to friend and training partner Joao Sousa.
Ramos-Vinola, ranked 27, and Portugal's Sousa, ranked 44, are both based in Barcelona and are supporters of the city's rival football clubs and Sousa described their match as "a kind of a derby."
"On court we are no friends but off court we are," he said.
Ramos-Vinola's main weapon is his powerful forehand but he lacked accuracy in his first match of the season, missing too many easy points in the first set and making 18 unforced errors to Sousa's eight. He dropped serve in the fourth game and again in the sixth to lose the first set in 32 minutes.