Roundup: Williams sisters semi on track at Australian Open

Venus Williams has joined her sister, Serena, in the Australian Open's final eight after ending the flawless run of Agnieszka Radwanska in the fourth round on Monday.

Meanwhile, men's world No.1, Novak Djokovic, had a challenging straight-set win over first-time opponent Gilles Muller to reach his 23rd straight Grand Slam quarter-final.

The Serb will next meet Milos Raonic, who earlier became Canada's first Australian Open quarter-finalist in 47 years.

In her 6-3, 2-6, 6-1 victory, Williams said she went "into a trance" after being broken in the opening game of the final set, allowing her to break the Radwanska serve thrice and catapult herself into the quarter-finals.

"I was just trying to get back the form I had in the first set," said the world No.18.

Radwanska had lost just nine games in her three previous matches at Melbourne Park and had rarely been troubled.

She found trouble immediately against the 18th seed and in an epic first-set game, Radwanska served 38 times - nine more than Williams for the entire first set - before conceding the decisive break.

The second set saw Radwanska higher in the court as she sought to fix the error of her first set and, after taking another break in the third, it seemed the wheels were falling off for her 34-year-old opponent.

Williams had a 5-4 head-to-head record against the Pole but had not tasted victory against the world No. 6 since better days in 2010.

But the seven-time Grand Slam champion brazenly refused to give up her 2015 unbeaten streak that started in Auckland and began discharging forehand torpedoes.

It destroyed the petite Pole's defences and, in a Samantha Stosur-like capitulation, Radwanszka won just two points on serve for the set.

"Things take time," said the five-time Wimbledon champion of her best Grand Slam performance for five years.

"Now is my moment and I want to keep this going all year and then next year too."

The Williams sisters remain on track for a semi-final showdown on the same court they fought out the 2003 title.

They last met in a Grand Slam in the 2009 Wimbledon final.

Before then, Serena must pass Dominika Cibulkova and Williams an opponent so young, the world No.18 conceded Madison Keys probably "started watching us when she was in diapers".

Keys, 19, is coached by one of Williams' greatest rivals, Lindsay Davenport, and will need all the tricks in the bag to upstage one of her idols in her maiden quarter-final.

In the men's draw, Djokovic flaunted his tricks but still needed gritty work to progress to the last eight for the eighth consecutive year.

The left-handed delivery from Muller made it tricky for the Serb.

"It was very hard to read his serve," said the seven-time Slam champion.

"His slice serve is excellent."

Djokovic's service was equally superb as both his accuracy on first serves and ability to convert them into winning moments ensured he was remained in the box seat throughout the 6-4, 7-5, 7-5 victory.

"I got the crucial three breaks at the important moments," said Djokovic, who last missed the final eight at a Grand Slam in 2009.

"There are a few things I think I can do better (but) overall, it's a positive feeling going into the quarters."

Djokovic, whose streak of 14 consecutive Grand Slam semi-finals was broken last year by tournament champion Stan Wawrinka, will next play Canadian No.1 Milos Raonic.

Eighth-seeded Raonic became the second Canadian to advance to the last eight in the Australian Open following Michael Belkin in 1968.

The Canadian tall timber did so by squeezing past Spanish veteran Feliciano Lopez 6-4, 4-6, 6-3, 6-7 (7-9), 6-3 to mark his third Grand Slam quarter-final.

Lopez, who scraped through his first two rounds before a straight-set win against Jerzy Janowicz, had won eight of his past nine five-set matches but the left-hander double-faulted on each break point he conceded and played himself out of it.

It makes Roger Federer the only of the top-eight men's seeds to miss the quarters, with his place taken by Australian Nick Kyrgios.

The women's draw is starkly different, with only Serena Williams (No.1 seed), Maria Sharapova (No.2), Simona Halep (No.3) and Eugenie Bouchard (No.7) surviving the first four rounds. Endi