The Serbian world number one, who is still yet to drop a set, once again looked in imperious form as he cruised to a 6-4, 7-5, 7-5 victory over Muller, who had beaten two seeds on his way to round for, to set up a meeting with Canadian hot prospect Milos Raonic.

Women's top seed Serena Williams also enjoyed a successful Australia Day as she battled back from a set down to beat Spain's Garbine Muguruza 2-6, 6-3, 6-2.

Muguruza shocked Williams at last year's French Open but there was to be no repeat this time around as the American shrugged off an illness to reach the next round.

"I had to play the best match of the tournament or else I was going to be out," she said.

"She was just hitting winners like left and right.

"Every shot I hit, she basically hit a winner on."

Serena will be joined in the quarter-finals by sister Venus, who will compete in the last eight of a Grand Slam for the first time since 2010.

The 33-year-old American also had to recover from a tough start before she eventually overcame Poland's sixth seed Agnieszka Radwanska in three sets, and she will meet compatriot Madison Keys, who earned a shock win over 2014 Wimbledon champion Petra Kvitova in round four, in the next round.

Venus Williams reached the quarter-finals of a Grand Slam for the first time since 2010 after she beat Agnieszka Radwanska in three sets ©Getty Images



On what was a relatively trouble-free day for the seeded players, Switzerland's men's defending champion Stanislas Wawrinka progressed with a four-set win over Spain's Guillermo Garcia Lopez, while Japanese fifth seed Kei Nishikori negotiated a tough test with consummate ease as he beat another Spaniard, David Ferrer, in straight sets.

Raonic, widely touted as a future Grand Slam winner, made difficult work of Spain's Feliciano Lopez as he squandered match points in the fourth set before he booked his quarter-final berth with a 6-4, 4-6, 6-3, 6-7, 6-3 win.

Elsewhere in the women's draw, Slovakia's 10th seed Dominika Cibulkova ensured she will face Venus Williams as she beat Belarusia's two-time Australian Open champion Victoria Azarenka 6-2, 3-6, 6-3.

The tournament is due to resume tomorrow with some mouth-watering clashes in both draws as Spain's Rafael Nadal takes on the Czech Republic's Tomas Berdych and Britain's Andy Murray faces home favourite Nick Kyrgios, while Canada's rising star Eugenie Bouchard plays Russia's second seed Maria Sharapova for a place in the semi-finals.

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