Unseeded Russian Ekaterina Makarova upset 12th seed and five-time champion Serena Williams of the United States 6-2, 6-3, to reach the quarterfinals of the Australian Open Monday.

The fourth-round match was played as temperatures hit above 32 degrees Celsius (90 degrees Fahrenheit).

Makarova's two previous Grand Slam appearances -- at Wimbledon and the U.S. Open last year -- ended with losses in the first round.

She will next meet the winner of the match between her compatriot, fourth seed Maria Sharapova, and 14th seed German Sabine Lisicki.

Earlier Monday afternoon, Wimbledon champion and world No. 2 Petra Kvitova reached the quarterfinals after beating Serbian 21st seed and former No. 1 Ana Ivanovic 6-2, 7-6 (2).

Kvitova will next face Italy's Sara Errani, who beat China's Jie Zheng 6-2, 6-1.

In other action, defending champion and 11th seed Kim Clijsters saved four match points, as she overcame the opponent she beat in last year's Australian Open final, Li Na, 4-6 7-6 6-4 on Sunday.

The former World No.1 was in trouble early on as she turned her ankle in the opening set and needed medical treatment before battling back to book a quarterfinal clash with world number one Caroline Wozniacki.

Fifth seed Li took full advantage of Clijster's discomfort by taking the first set 6-4 but the Belgian showed typical resilience when she leveled the match after a dramatic tie break, during which she faced match point four times.

The setback clearly stunned Li, who quickly found herself 5-2 down in the deciding set before staging a late rally. But the momentum was with the Belgian, as she closed out the match.

"I can't believe I won," Clijsters told the Australian Open's official website. "I knew before the match that it was going to be a tough match but I didn't expect this obviously!

"I don't want to quit my last time at the Australian Open. It [retiring] definitely crossed my mind at some point. But at one point you think 'Okay, I'm just gonna go for it.' Once I made that decision, I didn't think."

Wozniacki, from Denmark, beat Serbian 13th seed Jelena Jankovic 6-0 7-5 as she continues her quest to secure the first grand slam victory of her career.

After wrapping up the opening set in just over half an hour, Wozniacki dropped her serve twice in the second as Jankovic found her feet, but she held her nerve to serve out the match.

"I'm just happy to be here as the winner of the match," Wozniacki said. "I felt like I played some really good tennis out there today.

"She started to play really well as well. She had nothing to lose so she had to go for it. I knew she was a fighter and it wasn't going to be easy. "

Elsewhere Sunday, World No.3 Victoria Azarenka of Belarus cruised past Czech Iveta Benesova 6-2 6-2, to reach the last eight.

She will now face Agnieszka Radwanska, after the Pole thrashed Julia Goerges of Germany 6-1 6-1 earlier in the day.