- Serena Williams loses in straight sets to Russian world No. 56 in Melbourne
- Ekatarina Makarova earns quarterfinal clash with fourth seed Maria Sharapova
- World No. 2 Petra Kvitova reaches last eight after beating Ana Ivanovic
- Defending champion Kim Clijsters will face top seed Caroline Wozniacki in quarterfinal
Unseeded Russian Ekaterina Makarova upset five-time champion Serena Williams of the United States 6-2 6-3 on Monday to reach the quarterfinals of the Australian Open.
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 as she faded from a career-high 29th in the rankings -- a position she reached with last-16 appearances in Melbourne and at the French Open.
"I'm surprised because she's a great player and it's really tough to play against her," said the 23-year-old Makarova, who will next play her fourth-seeded compatriot Maria Sharapova.
"But, I don't know, I was just feeling so good and so focused. So I played my game, and that's it. I won against Serena. That's amazing."
Former world No. 1 Williams, seeded 12th, refused to blame the ankle injury she suffered in a warm-up tournament in Brisbane.
"I feel like I didn't play well today. I don't feel like I can't get better. I feel like if I was in a situation where, 'Wow, she played so great, and I played amazing, and I served 80%, and I hit no double-faults, I didn't hit 37 unforced errors,' that would be a totally different situation," the 30-year-old said.
"I didn't play well. I'm not physically 100%. So it's just like, you know, I can't be so angry at myself, even though I'm very unhappy. I know that I can play a hundred times better than I did this whole tournament."
Sharapova, the 2008 champion, secured her place in the last eight as she came from behind to beat German 14th seed Sabine Lisicki 2-6 6-2 6-3 in the late match.
It was their first meeting since last year's Wimbledon semifinals, when Sharapova also won before losing in the final.
Earlier Monday, 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 (7-2).
The Czech will next face Italy's Sara Errani, who beat China's Zheng Jie 6-2 6-1.
On Sunday, 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.
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 top-ranked Caroline Wozniacki.
Fifth seed Li took full advantage of Clijsters' discomfort by taking the first set, 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 dow