Defending champion Kim Clijsters battled back from a painful ankle injury to oust China's Li Na in three thrilling sets today as Rafael Nadal reached the quarter-finals with surgical precision.

Belgium's Clijsters looked on the verge of quitting when she rolled her left ankle in the first set, and despite gritting her teeth and playing on she looked down and out facing four match points in the second-set tiebreak.

But the despairing Li saw Clijsters save all four, culminating in a brave drop-and-lob combination against the flustered Chinese -- and the Belgian took the set as Li netted, and then went long.

It proved a dramatic tipping point as errors flowed from the racquet of French Open champion Li, who was broken immediately in the third set and could not claw her way back despite pleas from her coach and husband Jiang Shan.

Li recovered to break as Clijsters served for the match but the Belgian made no mistake on her next service game, clinching a famous win on her second match point when the Chinese player netted.

"It has to be," said a disbelieving Clijsters, when asked if it was her greatest comeback. "I can't believe I won. I knew before the match it was going to be a tough match... but I didn't expect this."

The result was a carbon-copy of last year's gripping final when Li, as the first Chinese player in a grand slam singles title match, took the first set but then fell to pieces as Clijsters came roaring back.

"It's amazing to get through. It probably wasn't my best tennis but you can see what happens if you want to fight, if you try to win each point and don't give up," said Clijsters.

The match ignited the first Sunday's action as packed-out Melbourne Park baked in heat of up to 35 degrees Celsius (95 degrees Fahrenheit). Elsewhere, Victoria Azarenka and Agnieszka Radwanska both had convincing wins.

And in men's action, Nadal removed fellow Spanish left-hander Feliciano Lopez to stay on course for a mouth-watering last-four clash with his great rival Roger Federer.

Nadal, seeking his 11th major victory and his second Open crown, shone in the afternoon sun as he won 6-4, 6-4, 6-2 to reach the quarter-finals without dropping a set.

The muscular second seed kept Lopez at bay with serving accuracy of 80 percent and he piled on the pressure with nearly error-free tennis to stay unbeaten on hardcourts against his Davis Cup team-mate.

"It was a very hot day and I think it's a positive to win in straight sets especially in these conditions," said Nadal, who will play now play Tomas Berdych after the Czech's four-set win over Nicolas Almagro.

The Spaniard remains on course to write the latest chapter in his epic rivalry with Swiss maestro Federer in the semi-finals. The all-time grand slam record-holder will face Australian teen Bernard Tomic on Sunday evening.

Meanwhile Azarenka defeated Czech Iveta Benesova 6-2, 6-2, and Radwanska brushed aside Germany's Julia Goerges 6-1, 6-1.

Azarenka has dropped only 12 games in her four matches and was quickly into her stride against her 46th-ranked opponent, breaking twice in the first set.

She maintained her firm grip on the contest in the second set, again breaking twice to wrap up the match in 1hr 17mins.

Azarenka, who is in contention to secure the number one ranking after the Australian Open, said she felt full of confidence after her win in Sydney earlier this month, which means she is still unbeaten this year.

Eighth seed Radwanska, 22, needed just 54 minutes to reach the quarter-finals for the second year in a row. She made just three unforced errors in the match against 27 from Goerges.