Sydney: A tough playing schedule caught up with world number six Agnieszka Radwanska when she was dumped out in the first round of the Sydney International on Tuesday.
Spain`s Garbine Muguruza, who won last year`s Hobart International as a qualifier and is currently ranked 24, fought back after dropping the opening set to beat the Polish third seed 3-6, 7-6 (7/4), 6-2.
It is the first tournament that Radwanska, coming off Poland`s win in the mixed teams Hopman Cup in Perth, is playing under new coach and tennis great Martina Navratilova.
"(Tired) a little bit, yes, especially that last couple matches was pretty long," Radwanska said of her recent Hopman Cup campaign.
"Plus the mixed doubles and not much rest and I was playing here the first round. So I think just everything in (a) row.
"I think she (Muguruza) has really good potential, and I think every day she`s playing really good tennis, better tennis, and definitely the tennis that we going to see much more of in the future."
Muguruza, who lost her previous two matches with Radwanska, said the win showed how much she has improved her tennis.
"For me to win this match, I feel like I improve a lot," she said. "When I play against her I really have to concentrate and do my game, because she makes me play different styles and dropshots and long balls."
Two-time Wimbledon champion and Czech second seed Petra Kvitova was too strong for China`s Peng Shuai, winning 6-1, 7-5.
Fellow Czech Barbora Zahlavova Strycova knocked out Australia`s top-ranked player Samantha Stosur 7-6 (7-1), 6-2 and will face defending champion Tsvetana Pironkova in the next round.
Pironkova won her 13th straight match at the Sydney tournament, progressing on an injury retirement into quarter-finals.
Pironkova advanced when American youngster Madison Keys was forced to retire from their second-round encounter with a right shoulder injury.
Keys had won the opening set 6-4 but trailed 4-1 in the second before calling it quits.
The Bulgarian continued on her winning way and remained on track to win the trophy as a qualifier for the second successive year.
Spain`s Carla Suarez Navarro ousted Russian sixth seed Ekaterina Makarova 4-6, 6-1, 6-4 to progress to a quarter-final against the Czech Republic`s Karolina Pliskova.
Pliskova took full advantage of the withdrawal of Romanian top seed Simona Halep to trounce American Nicole Gibbs 6-0, 6-0.
Halep said she was forced out with gastroenteritis she picked up during her tournament win in Shenzhen in China last week.