SYDNEY (AFP) - A tough playing schedule caught up with tennis' world No. 6 Agnieszka Radwanska when she was dumped out in the first round of the Sydney International on Tuesday.

Spaniard Garbine Muguruza, who won last year's Hobart International as a qualifier and is currently ranked 24, fought back from dropping the opening set to beat the Polish third seed 3-6, 7-6 (7-4), 6-2.

It is the first tournament Radwanska, coming off Poland's win in the mixed-team 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," she 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."

Defending champion Tsvetana Pironkova won her 13th straight match at the Sydney tournament progressing on an injury retirement into quarter-finals.

She 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.