Novak Djokovic kept his hopes of a third successive Australian Open title alive after emerging victorious from an extraordinary five-hour epic match against Stanislas Wawrinka.
The world number one came through 1-6 7-5 6-4 6-7 (5/7) 12-10 in five hours and two minutes of absorbing tennis against an opponent who was inspired.
Having withstood an early Wawrinka barrage it seemed the momentum had turned in the Serbian's favour when he claimed sets two and three.
But back came Wawrinka to win the fourth in an epic tie-break to take it to a decider which could have gone either way until Djokovic finally made the breakthrough with the match finishing at 1.43am local time.
"It's really hard to find the words to describe the feeling we had tonight, especially in the fifth set," said Djokovic.
"He equally deserved to be a winner of this match. I give him a lot of credit, he has my respect. He showed his quality and was the aggressive player on the court."
Agnieszka Radwanska continued her perfect start to the year with a clinical demolition of Ana Ivanovic to set up a rematch with Li Na in the quarter-finals..
The number four seed at Melbourne has not dropped a set this year, racking up thirteen straight victories - including today's 70-minute rout of the former world number one.
"Well, what can I say? I'm just very happy that I can play at a very high level," she said afterwards.
"I hope I can keep going three more."
The 23-year-old Pole joked before the match that she was not superstitious about the number 13 and needed precious little luck in her 6-2 6-4 rout of the former world number one.
Radwanska made just four unforced errors to the wayward Serbian's 34 and saved five break points in the second set before sealing victory with her seventh ace after 70 minutes.
Her frenetic all-court game on a cool summer evening at Rod Laver Arena left Ivanovic in little doubt the Pole could go all the way at Melbourne Park.
"I think she definitely can," the former French Open said ater the match.
"She's a player, it's no secret, who doesn't make too many winners, but she has good hands. She can really get the opponents to feel a little awkward make you go for too much.
"That's her strength."
Ekaterina Makarova sprung a second successive fourth round upset after completing a 7-5 6-4 victory over fifth seed Angelique Kerber.
The 19th seed survived a wobble to close out the first set before going on to defeat her injury-hampered German opponent, the highest seed so far to lose in the women's draw, in one hour and 32 minutes with an ace.
Makarova advanced to her second consecutive quarter-final at Melbourne Park, following last year's tournament where she defeated five-time champion Serena Williams.
"It's an unbelievable feeling," the 24-year-old Muscovite said in a courtside interview at a sunbathed Rod Laver Arena.
"I really like to play here, everything is so perfect. My team did a really nice job."
Maria Sharapova continued her progress through the Australian Open with a 6-1 6-0 victory over Belgium's Kirsten Flipkens.
Sharapova had been the most dominant player in the women's draw so far, recording successive 6-0 6-0 victories in the first two rounds before disposing of Venus Williams in the third round.
The second seed, however, took 43 minutes to record a 6-1 first set won over her tenacious Belgian opponent.
Sharapova cranked into gear in the second set to clinch it in 25 minutes and set up a quarter-final against fellow Russian Ekaterina Makarova after the 19th-seed dispatched fifth seed Angelique Kerber 7-5 6-4.
Big-hitting Czech Tomas Berdych is determined not to let missed opportunities ruin his chances of going further than he has ever gone before at Melbourne.
The fifth seed survived an epic third-set tiebreaker against Kevin Anderson, winning it 15-13 to clinch a 6-3 6-2 7-6 victory.
Berdych said he was determined only to look forward and not back after setting up a meeting with either Novak Djokovic or Stanislas Wawrinka, the number 15 seed.
"I would say I feel much better," the Czech said. "The last year was a pretty big year for me. There were many new experiences that I learnt. I hope that I'm going to be able to profit from that.
"Even to play the semi-final in the US. Open, that match with Andy Murray, even though it didn't go well, I could learn a lot from that."
Nicolas Almagro will meet fellow Spaniard David Ferrer in the quarter-finals after Janko Tipsarevic retired hurt in their match.
The eighth seeded Serb received treatment on a foot injury before withdrawing while trailing 6-2 5-1 in the match at Hisense Arena.
The 10th-seeded Almagro advances to his first quarter-final at Melbourne Park in his ninth appearance at the year's first grand slam after making the last eight at the French Open three times.
Ferrer, meanwhile, exploited opponent Kei Nishikori's nagging knee injury to advance 6-2 6-1 6-4 into the match with Almagro.
Nishikori needed to have his left leg re-strapped in the second set of the two hour, 10-minute match, and his normally fluid movement appeared to be slightly hampered.
Ferrer took full advantage of the 16th seed's injury, working him around the court and waiting for the 23-year-old to make one of his 65 errors, though the Japanese number one did put up some resistance in the third set.
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