(CNN) -- 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 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 Polish eighth seed Agnieszka Radwanska, who thrashed Julia Gorges of Germany 6-1 6-1.
CNN's Elizabeth Yuan contributed to this report.