Australian Open: Djokovic battles on, Nishikori beats Carreno Busta in 5-set epic

Melbourne: A fired-up Serena Williams slugged past world number one Simona Halep on Monday as she inches ever closer to a 24th Grand Slam title, while Novak Djokovic survived a battle to stay on course for a record seventh Australian Open crown.

In a towering performance, the American great scored her biggest win since having a baby to grind down the Romanian top seed 6-1, 4-6, 6-4 and move into a quarter-final against seventh seed Karolina Pliskova.

Czech Pliskova is another form player, winning the warm-up Brisbane International and easing past two-time Grand Slam winner Garbine Muguruza 6-3, 6-1 to set up the Williams clash.

"I'm such a fighter, I never give up," declared Williams after the epic encounter.

"It was an intense match and there were some incredible points but I love playing tennis and being out here and I love this court, it's really cool to be back."

While Williams stayed calm, highly-rated German fourth seed Alexander Zverev lost the plot against Canadian Milos Raonic, crashing 6-1, 6-1, 7-6 (7/5) and demolishing a racquet in a massive meltdown.

And Spain's Pablo Carreno Busta also let the pressure get to him, ranting at the umpire after a disputed call when leading the final tiebreak 8-5 against Japan's Kei Nishikori.

He failed to win another point and was booed as he stormed off Margaret Court Arena, angrily throwing his kit bag after losing a 5hr 5min marathon 6-7 (8/10), 4-6, 7-6 (7/4), 6-4, 7-6 (10/8).

Nishikori will now play six-time champion Djokovic, who dropped a set for the second match in a row before seeing off 15th-seeded Daniil Medvedev 6-4, 6-7 (5/7), 6-2, 6-3.

"It was definitely a physical battle. It was difficult to get through him," the Serbian world number one said. "In these kinds of matches you just have to hang in there."

Elevate game

Dominant Williams overshadowed them all and is overwhelming favourite to win an eighth Australian title, with defending champion Caroline Wozniacki and second seed Angelique Kerber already out.

She has been in ominous form at Melbourne Park as she seeks one more major title to put her alongside Margaret Court's record 24, and it looked like French Open champion Halep would be another easy scalp after a first set rout.

But the nuggety Romanian isn't world number one for nothing and she bounced back to set up a huge power struggle that Williams finally won with a break in the seventh game of the third set.

"I really needed to elevate my game. She's a great player," said Williams.

Fourth seed Naomi Osaka, meanwhile, was composed as she beat crafty Latvian Anastasija Sevastova, emerging from an error-strewn first to win 4-6, 6-3, 6-4.

"I think the biggest thing for me is the belief. I think I believe in myself more this year than I have last year," said the bubbly 21-year-old, who beat Williams in the US Open final last year.

She next faces ambitious sixth seed Elina Svitolina, who needed to dig deep to overcome 17th seeded Madison Keys 6-2, 1-6, 6-1.

Svitolina was a quarter-finalist at Melbourne Park last year and has set her sights high after her breakthrough in winning the WTA Finals Singapore, targeting a Grand Slam win and world number one.

Earlier on Rod Laver Arena, Zverev self-destructed to slump out at the hands of Raonic.

The German has routinely been anointed as the flag-bearer for the next generation, but again failed to live up to his reputation and has now missed out on making the last eight of a major 14 times in 15 attempts.

So furious was he with his form, the 21-year-old had a major temper tantrum in the second set, whacking his racquet into the ground eight times to leave it a crumpled mess.

"I just tried to stay composed and it worked out well for me today," said cool 16th seed Raonic, who will now face Frenchman Lucas Pouille after he battled past Croat Borna Coric in four tough sets.