Serbia's Novak Djokovic celebrates after defeating Canada's Milos Raonic during their fourth round match at the Australian Open tennis championship in Melbourne, Australia, Feb 14, 2021. (HAMISH BLAIR / AP)

MELBOURNE - Reigning champion Novak Djokovic came through a tough examination of his fitness as the top seed beat Canadian powerhouse Milos Raonic 7-6(4) 4-6 6-1 6-4 on Sunday to reach the Australian Open quarter-finals.

Djokovic, who injured a side muscle during a five-set win over Taylor Fritz on Friday and suggested he might not be able to continue, showed hints of discomfort on the way to becoming the second male player to reach 300 Grand Slam match wins.

Fourteenth seed Raonic had never previously taken a set off Djokovic in three previous Grand Slam meetings so when he snatched the second set a surprise looked possible.

It’s not ideal, I’ve felt better that’s for sure. Didn’t have much preparation and didn’t hit a tennis ball since two nights ago. There was a cloud of doubt whether I would play but it’s a Grand Slam and I gave everything to have a chance.

Novak Djokovic, World number 1 Serbian tennis player

But Djokovic, bidding for a record-extending ninth title in Melbourne, responded by raising his level and improving his record over Raonic to 12-0.

The Serb will face Germany’s Alexander Zverev for a place in the semi-finals.

After the drama of Friday when Djokovic said he had sustained a “tear” and could not move, this was a relatively routine day at the office for the world number one who has been on painkillers for 48 hours.

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“It was good enough to compete, to play, I wouldn’t be standing here talking if I wasn’t in sufficient condition to get on the court and give myself a chance to win a match,” Djokovic told Eurosport.

“It’s not ideal, I’ve felt better that’s for sure. Didn’t have much preparation and didn’t hit a tennis ball since two nights ago. There was a cloud of doubt whether I would play but it’s a Grand Slam and I gave everything to have a chance.”

Canada's Milos Raonic hits a backhand return to Serbia's Novak Djokovic during their fourth round match at the Australian Open tennis championship in Melbourne, Australia, Feb 14, 2021. (HAMISH BLAIR / AP)

Djokovic failed to convert break points at 3-3 and 4-4 in the opening set and there was one moment of alarm when he was forced wide on the forehand side and struggled to put on the brakes, hurdling a courtside advertising board.

Clinical fashion

He won the tiebreak in clinical fashion, though, and when Raonic needed treatment on his right ankle early in the second set Djokovic appeared to be well in command.

Instead, Raonic began to play some of his best tennis and secured the first service break of the match at 2-2 with a powerful forehand winner.

That proved enough for Raonic to bag the second set but his victory hopes were soon receding as Djokovic began to swing freely and accelerated through the third set.

The elastic-limbed Djokovic occasionally looked tentative when moving out wide on the forehand side and was stretched in the fourth set but he broke Raonic’s serve at 4-4 when his opponent netted a backhand and clinched victory a game later to reach the quarter-finals in Melbourne for the 12th time.

Serbia's Novak Djokovic hits a forehand return to Canada's Milos Raonic during their fourth round match at the Australian Open tennis championship in Melbourne, Australia, Feb 14, 2021. (HAMISH BLAIR / AP)

His 300th Grand Slam match win put him behind only Roger Federer (362) in the men’s list.

Djokovic will face a potentially tougher challenge against Zverev and said his immediate focus would be on recovering.

“I just want to feel better, I don’t know exactly the damage that is being done,” he said.

“In the next 48 hours it’s about recovering as much as possible and collect all the energy for the next match.”

Germany's Alexander Zverev celebrates his win over Serbia's Dusan Lajovic in their fourth round match at the Australian Open tennis championships in Melbourne, Australia, Feb 14, 2021. (ANDY BROWNBILL / AP)

Easy win over Lajovic for Zverev

Earlier on Sunday, German sixth seed Alexander Zverev used his powerful serve to good effect to overcome 23rd seed Dusan Lajovic of Serbia 6-4 7-6(5) 6-3 and reach the Australian Open quarter-finals.

Zverev needed five sets to beat Lajovic in each of their two previous meetings at the French Open, but the 23-year-old was in a more ruthless mood on Margaret Court Arena and sealed victory with his 15th ace.

Alexander Zverev got 72 percent of his first serves in and racked up 44 winners to dominate Dusan Lajovic throughout the match to register his 50th win in majors

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The German got 72 percent of his first serves in and racked up 44 winners to dominate the 30-year-old Lajovic throughout the match to register his 50th win in majors.

Lajovic started strongly by breaking the German early to take a 2-0 lead in the opening set but Zverev broke back in the next game and did not look back.

“Today was a pretty good match, much colder than the other days,” Zverev said in his on-court interview.

“I’m very happy with a win and it could have gone either way early on. It wasn’t easy.”

“I’m in the quarter-finals of a Grand Slam, so I’ve gotta to be doing something well,” Zverev said.