Novak Djokovic and Serena Williams faced what were supposed to be their biggest challenges to date at this year's Australian Open on Tuesday but both rattled off comfortable victories to march into the semi-finals.

Six-time champion Serena Williams advanced to the semifinals yet again, extending her dominance in a 12-year rivalry with an 18th straight win over Maria Sharapova.

Sharapova won more sets against Williams in three matches in 2004 (four) than in their 18 meetings since (three).

The world number one's incredible record against Sharapova dates back to an epic semi-final at the 2005 Australian Open, when she scraped home in a 2-6, 7-5, 8-6 thriller. "He's one of those guys who make you a better player, he's beaten me on the biggest courts around the world".

Serena Williams is coming off a straight sets win over Maria Sharapova and hasn't dropped a set all tournament.

An error-strewn service game from Williams saw her broken immediately and raised the prospect of a thrilling contest, however back-to-back double faults from Sharapova allowed her opponent to level shortly afterwards. "Yeah", Sharapova added in the same report.

He beat Federer in the Wimbledon and U.S. Open finals past year, but Federer was back contending for titles.

"It's motivating", Sharapova said of her loss to Williams.

"She is very solid, like most Spanish players and I knew everything would come back to my side". But Williams soon dominated the third game and broke to quickly level at 2-2.

Early breaks were enough to secure the first two sets for the Serbian, who later revealed he had not hit a single ball on Monday, and only after an injury timeout did Nishikori rediscover the aggression that got him to the last eight.

But that approach is what has helped win her 21 major titles, and Williams' aggressive returns finally helped her convert on her fourth set point, following a heavy ground stroke to the net and putting away a volley.

Williams broke for a second time to take the first set in 55 minutes but it was clear something was not quite right with the powerful American.

In the women's semifinals, Williams will be facing fourth-seeded Agnieszka Radwanska, who beat No. 10 Carla Suarez Navarro 6-1, 6-3.

Catch up on all of the action, news and results from the first day of quarterfinals matches at the 2016 Australian Open on Tuesday that you may have missed while you were sleeping.

The only potential cloud on the horizon for Williams at this stage of the tournament is her health.

It's the second semi-final Radwanska's made at Melbourne Park, and she said it wasn't as easy as the scoreline would suggest.

Williams is gunning for a seventh Aussie Open title, which would tie Steffi Graf's record of 22 Grand Slam titles. "It's part of the reason why I'm still playing", Federer said.

"The last few days were very, very tough".

"Doesn't matter who I'm playing, I just try to go out there and play the best I can", Williams said.