MELBOURNE (AFP) - Seven-time Grand Slam champion Venus Williams stormed into the Australian Open quarter-finals Monday, overcoming fatigue to roll back the years and down sixth seed Agnieszka Radwanska in three gruelling sets.

The 34-year-old American finished strongly to win 6-3, 2-6, 6-1, sealing match point with an ace to reach the final eight of a major for the first time since the US Open in 2010.

Cheered on from the stands by top-seeded sister Serena, an exhausted Venus somehow found the energy to get past highly rated Radwanska of Poland and set up a quarter-final with unseeded compatriot Madison Keys.

"In the third I think I went into a trance. I just wanted to win," the ecstatic 18th seed said.

The win takes her 2015 record to 9-0, including winning the Auckland Classic lead-up event.

It continues a remarkable comeback from injury and a long battle with the energy-sapping Sjogren's Syndrome, which was diagnosed in 2011.

The lanky American looked like the Venus of old early in the match, covering the court with ease to blast venomous returns and pin-point accurate winners.

She clearly flagged as the match wore on, clocking up 36 unforced errors but hung on grimly as Radwanksa extended the rallies and ran her around.

She moved briskly in the first set, notching a hard-earned break in the seventh game that stretched more than 12 minutes and eventually taking it in just under an hour.

Radwanska hit back in the second set, benefitting from several basic mistakes when her opponent fluffed opportunities with an open court in front of her.

The turning point came after they exchanged breaks early in the third then Venus somehow wrested back momentum to break again in the fourth game. She went on the bring up three break points but only needed one as she blasted her sixth ace of the match.

Venus will meet unseeded 19-year-old compatriot Madison Keys in her quarter-final match. Keys, the world no. 35, beat fellow American Madison Brengle 6-2, 6-4 earlier in the day.