Maria Sharapova came from a set down to beat Sabine Lisicki and will now meet Ekaterina Makarova who knocked out Serena Williams in straight sets. Jo-Wilfried Tsonga also crashes out but Andy Murray breezes through.
Australian Open latest: Updates from the first tennis grand slam of the
Lleyton Hewitt rolled back the years tonight to deliver a typically gutsy display but it was not enough to derail Novak Djokovic's title defence.
Hewitt looked down and out at two sets and 3-0 down but he stormed back to take the contest to a fourth set in which the world No 1 showed battling qualities of his own to edge through 6-1 6-3 4-6 6-3 in nearly three hours.
"I have to give credit to my opponent who never gives up," said Djokovic.
"He's a great competitor and he kept on making me play an extra shot. I made a couple of unforced errors and he got back into the match. I have a lot of respect for him.
"For two sets and 3-0 I was playing really well and then I stopped moving but credit to him, he wasn't making any unforced errors."
Next for Djokovic is a meeting with fifth seed David Ferrer in the last eight.
Serena Williams has lost at the Australian Open for the first time since 2008, struggling with her serve and hitting too many unforced errors in a shocking 6-2, 6-3 fourth-round defeat to Ekaterina Makarova.
Williams was surprised by the power of the groundstrokes coming back at her Monday from the Russian lefthander, who at No. 56 was the lowest-ranked woman to make the fourth round of the season's first major.
The dominant force at Melbourne Park this century, Williams had lost only two matches at the Australian Open since winning the first of her five titles here in 2003.
But she had seven double faults - including four in the fifth game of the second set - and 37 unforced errors to give Makarova a spot in the quarterfinals at a major for the first time. She'll play 2008 champion Maria Sharapova.
"I don't know what to say. Amazing feeling and first time in quarterfinals," the 23-year-old Makarova said. Williams is "an unbelievable player. It's really tough to play against her so I'm really happy I finished it in my way."
Williams sprained her left ankle in a warmup tournament at Brisbane two weeks ago, but didn't show any signs of being restricted on Monday.
She was bothered by a bug that landed on her left shoulder when she dropped serve for the first time in the match, and became increasingly exasperated as her misses piled up - including one overhead that she sent way too long and another that she hit meekly back for Makarova to pass her.
Williams won the first two games in the second set but then Makarova went on a roll, winning the next four games - including the double-fault strewn game at 2-2 when Williams screamed after one and asked herself out loud after another: "How many double-faults do you want to make?"
The Russian got tighter toward the end but kept her nerve to hold in a key game. Then, with Williams serving to stay in the match, she needed four match points before Williams sent a backhand wide.
Sharapova needed all of her experience to overcome a determined Sabine Lisicki, 3-6 6-2 6-3, to advance to the quarter-finals.
The fourth-seeded Russian won the opening three games but suddenly lost her mojo as the German reeled off the next six games to seize the first set and seemingly all the momentum. But Sharapova fought back in the second set and then scrapped tooth and nail in the third game of the third to hold serve before she broke 14th seed Lisicki in the sixth, which ultimately setting up her victory. Sharapova will next meet countrywoman Ekaterina Makarova after the world No 56 caused the upset of the tournament by thrashing five-times champion Serena Williams 6-2 6-3.
Kei Nishikori shocked former finalist Jo-Wilfried Tsonga in five sets to become the first Japanese man into the Australian Open quarter-finals in 80 years on Monday.
The 24th seed dug deep to knock out sixth seed Tsonga, 2-6, 6-2, 6-1, 3-6, 6-3 in 3hr 30min in sweltering conditions on Hisense Arena.
It was Nishikori's second win over Tsonga after beating him in Shanghai last year and sets him up with a quarter-final against British fourth seed Andy Murray on Wednesday.
It is the first time a Japanese man has reached the last eight at the Australian Open since 1932, when there were two in the quarter-finals.
It was Nishikori's best result at a grand slam, surpassing his fourth round appearance at the 2008 US Open.
He also became only the second Japanese man to reach a grand slam quarter-final in the open era since Shuzo Matsuoka at Wimbledon in 1995.
"I was just playing one point at a time and it was not easy conditions today, Jo-Wilfried was playing well and I was just trying my best," Nishikori said.
"Hopefully it's big news in Japan. A lot of people have messaged me since my last win, so now it's a quarter-final and I'm really excited."
Andy Murray said he would need to hit the practice courts to retain his sharpness after his "boring" stroll into the quarter-finals following an injury retirement at the Australian Open on Monday. The British fourth seed was on court for only 49 minutes before his Kazakh opponent Mikhail Kukushkin retired early in the third set with a hip flexor injury.
Murray, who will face either French sixth seed Jo-Wilfried Tsonga or Japan's 24th seed Kei Nishikori in the quarter-finals, said he was bored by his one-sided match with Kukushkin which he won 6-1, 6-1, 1-0.
"Yeah, it's just boring. There was nothing happening on the court," Murray said of the match against his 92nd-ranked opponent.
"I didn't have to do anything. Just hitting the ball in the court and he wasn't running. He was making mistakes the first or second ball of the rally," Murray added,