American teenager Madison Keys toppled fourth-seed Petra Kvitova 6-4 7-5 in an impressive display of clean hitting to reach the fourth round of the Australia Open.

The 19-year-old captured the decisive break at 5-5 in the second set and served out the match nervelessly under the lights of Rod Laver Arena, stunning the two-time Wimbledon champion.

Top-seed Serena Williams and her older sister Venus sent a chilling warning to the rest of the women's draw by advancing to the fourth round at Melbourne Park for the first time in five years.

Both took a little while to figure out their opponents on Saturday but once they got into the groove of the match were able to advance relatively comfortably.

World number one Serena beat Ukraine's Elina Svitolina 4-6, 6-2, 6-0, while 18th-seed Venus also needed a set to get going before she beat Italy's Camila Giorgi 4-6, 7-6, 6-1.

Women's sixth-seed Agnieszka Radwanska also continued her largely untroubled, yet under the radar, progress through the draw with a 6-0, 7-5 win over Varvara Lepchenko and will meet Venus on Monday.

Two-time champion Victoria Azarenka won a battle of former girls' champions at the Australian Open by easing past Czech Barbora Zahlavova Strycova 6-4, 6-4.

Azarenka will play last year's finalist Dominika Cibulkova for a place in the quarter-finals.

Source: Reuters