Venus Williams advanced into her first grand slam quarter-final since the 2010 US Open with an upset 6-3, 2-6, 6-1 victory over sixth-seed Agnieszka Radwanska at the Australian Open.

The 34-year-old, who suffers from Sjogren's syndrome, an auto-immune disease that can cause fatigue, was forced into a lengthy baseline battle with one of the best retrievers in the women's game.

Williams, however, put pressure on Radwanska's serve throughout, illustrated by the seventh game of the first set which lasted 15 minutes with the score locked at deuce 12 times before the American converted her sixth break point.

Radwanska appeared to have taken Williams' legs out from her in the second set before the 18th seed found a second wind in the decider, jumping out to a 4-1 lead as the Pole's accuracy disintegrated, then breaking to love and serving out to set up a quarter-final against fellow American Madison Keys.

Earlier, sister Serena Williams took some coaching advice from a fan sitting in the Rod Laver Arena terraces as she struggled to overcome hard-hitting Spaniard Garbine Muguruza 2-6, 6-3, 6-2 to also reach the quarter-finals.

She will play last year's losing finalist Dominika Cibulkova for a place in the semi-finals, after the pint-sized Slovakian dumped out two-time champion Victoria Azarenka 6-2, 3-6, 6-3.

Results:

18-Venus Williams (U.S.) beat 6-Agnieszka Radwanska (Poland) 6-3 2-6 6-1

Madison Keys (U.S.) beat Madison Brengle (U.S.) 6-2 6-4

1-Serena Williams (U.S.) beat 24-Garbine Muguruza Blanco (Spain) 2-6 6-3 6-2

11-Dominika Cibulkova (Slovakia) beat Victoria Azarenka (Belarus) 6-2 3-6 6-3

Source: Reuters