Doha, Qatar (SportsNetwork.com) - Australian Open champion Li Na suffered a third-round upset at the $2.44 million Qatar Open.

Czech qualifier Petra Cetkovska stunned the top-seeded Chinese star in 7-6 (7-2), 2-6, 6-4 fashion in 2 hours, 46 minutes on the hardcourts at Khalifa International Tennis Complex. Playing her first tennis since capturing her second Grand Slam title last month, Li blew a 3-1 lead in the third set on Thursday.

This marks the eighth time in her career that Cetkovska has beaten a top-10 player.

Meanwhile, second-seeded Pole and former Wimbledon runner-up Agnieszka Radwanska cruised past Croatian qualifier Mirjana Lucic-Baroni 6-4, 0-1, retired; third-seeded Czech slugger Petra Kvitova outlasted fellow countrywoman Lucie Safarova 7-6 (7-2), 5-7, 6-2; fourth-seeded former French Open runner-up Sara Errani of Italy overcame Romanian Monica Niculescu 2-6, 7-5, 6-1; and fifth-seeded former world No. 1 Jelena Jankovic of Serbia subdued Russian wild card Alisa Kleybanova 6-1, 6-2.

Also on Day 4, sixth-seeded German left-hander Angelique Kerber tackled Czech veteran Klara Zakopalova 6-2, 6-3, seventh-seeded Romanian Simona Halep humbled German Annika Beck 6-4, 6-1, and Belgian Yanina Wickmayer overcame Slovak Jana Cepelova 1-6, 6-2, 6-0.

Friday's quarterfinals will pit Radwanska against Wickmayer, Kvitova versus Jankovic, Errani against Halep, and Kerber versus Cetkovska.