Venus Williams, who once ruled the tennis world, has risen from her recent relative obscurity to make the quarter-finals of a major championship for the first time in five years.

Williams, 34, will play fellow American Madison Keys, who was five years old when Williams won the first of her five Wimbledon crowns and is now a tender 19.

Venus downed sixth seed Agnieszka Radwanska in three gruelling sets, 6-3 2-6 6-1, sealing match point with an ace.

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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.

“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 is the first time she has made the quarter-finals of a Grand Slam tournament since the 2010 US Open.

Madison Keys will play Venus Williams, 15 years her senior. Photo: Getty

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 pinpoint 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.

The talented Keys advanced to her first major quarter-final with a comprehensive 6-2 6-4 victory over American Madison Brengle, dominating the winners’ count over her compatriot 38-3.

Venus and Keys are, respectively, the oldest and youngest quarter-finalists in the women’s draw.

Venus is also on the same side of the draw as Serena, with a possible semi-final clash looming between the sisters.

Serena also progressed in three sets, defeating Garbine Muguruza 2-6 6-3 6-2 in a match in which the world No.1 took tactical advice from her fans in the stands.

Down a set and struggling against free-swinging Spaniard, a spectator at Rod Laver Arena yelled out, advising Williams to readjust and use more spin.

Serena could player sister Venus in the semi-finals. Photo: Getty

The American took the comment on board and turned the fourth-round encounter around to advance to a quarter-final against last year’s Open runner-up, Dominika Cibulkova from Slovakia.

“My fan coach was yelling out ‘use some spin Serena’ and I was like `okay, okay’,” said Williams, who remains on track for a sixth Australian Open title and a first since 2010.

“But I get that (level of support) every year in Australia and that’s why this is my favourite place to come. I really love it so much.”

Williams and the 24th-seeded Muguruza had split their previous two encounters – both of which were one-sided, in sharp contrast to Monday’s two-hour marathon.

The American had also dropped the first set of her third-round clash against Ukraine’s Elina Svitolina.

For the second straight year, the diminutive Cibulkova is also making a run deep into the Open, the No.11 seed from Slovakia charging into the last eight with a 6-2 3-6 6-3 victory over two-time Open champion Victoria Azarenka.

After battling injuries and personal issues in 2014, the Belarusian entered the first grand slam of the year without a seeding, although she shaped as the most dangerous floater in the draw.

But it was Cibulkova who triumphed, winning in two hours and 10 minutes. Cibulkova’s best-ever performance at a grand slam came last year at Melbourne Park when she advanced all the way to the final before losing to China’s Li Na.

In the men’s draw, three Spaniards were beaten. Canadian Milos Raonic defeated Feliciano Lopez, defending champion Stanislas Wawrinka won in four sets against Guillermo Garcia-Lopez and Japanese star Kei Nishikori won in straight sets against David Ferrer.

In the final match, tournament favourite Novak Djokovic defeated Luxembourg’s Gilles Muller in straight sets to become the last of the quarter-final qualifiers.

To see the quarter-final line-up, click on the owl. 

– with AAP