Serbia's Novak Djokovic runs to hit a shot during his quarter-final match against Japan's Kei Nishikori at the Australian Open tennis tournament at Melbourne Park, Australia, Jan 26, 2016. [Photo/Agencies]

MELBOURNE - 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.

Australia Day fireworks were expected on court and off on day nine of the championships but Djokovic extinguished Kei Nishikori's flame 6-3 6-2 6-4 and Williams doused every spark of a threat from Maria Sharapova to win 6-4 6-1.

The defending champions face very different hurdles in the last four, however, with Djokovic moving on to face 17-times grand slam winner Roger Federer and Williams's path to a seventh final blocked by the slight figure of Agnieszka Radwanska.

Federer eased past Tomas Berdych 7-6(4) 6-2 6-4 while fourth seed Radwanska thrashed Carla Suarez Navarro 6-1 6-3 to reach her second semi-final at Melbourne Park.

Djokovic has played five and had already booked a place in his sixth when the traditional festive fireworks lit up the skies over Melbourne to celebrate the national holiday.

The 28-year-old, supposedly vulnerable to an upset after racking up 100 unforced errors in a five-set marathon against Gilles Simon in the fourth round, rolled on remorselessly.

Seventh seed Nishikori stunned the world number one the last time they met in a grand slam at the 2014 U.S. Open, but there was to be no repeat.

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.

Asia's number one raced to a 3-1 lead on the back of two breaks in the third set but Djokovic reined him in with two breaks of his own and grabbed a third before finishing the Japanese off with a crunching backhand winner after just over two hours.

"I was solid," Djokovic said. "I was determined, focused. In important points and moments I managed to stay composed and make him play an extra shot.

"Overall it was a very solid performance."