MELBOURNE－Novak Djokovic says it's a "gamble" to keep playing the Australian Open and his abdominal injury could have an impact on the rest of his season, but the world No 1 is prepared to take the risk.

The Serbian eight-time champion sustained a "muscle tear" during a thrilling five-setter against Taylor Fritz on Friday.

He didn't train the following day but took to Rod Laver Arena Sunday evening dosed up on painkillers to see off the threat posed by big-serving Canadian Milos Raonic in four sets to make the last eight.

Djokovic said the pain was "bearable" and he "somehow managed to find a way and win".

But he is not planning to train again on Monday, instead working on his recovery ahead of his last-eight clash with sixth seed Alexander Zverev.

"I mean, it's kind of a gamble, that's what the medical team told me. It's really unpredictable, you can't know what's going to happen with you once you're on the court," Djokovic said.

"You're not gonna save yourself or think about going for that point or this shot or that shot. It just pulls you. It's normal. Playing at this level, you just want to give it your all.

"It could cause much more damage than it is at the moment, but also could go in a good direction."

The 33-year-old refused to say what exactly the problem was, although his abdomen was strapped against Raonic and he said after the Fritz clash that it was "a tear of the muscle".

"I know what it is, but I don't want to talk about it now. I'm still in the tournament," he said, conscious that he does not want to give Zverev any extra ammunition to target him.

"I hope you guys understand that. I don't want to speculate too much about it."

Djokovic's win against Raonic was his 300th at a Slam, only the second player in history to reach the mark after Roger Federer, who has 362.

It kept him on track for an 18th Grand Slam title in his bid to close in on the 20 held by the Swiss great and Rafael Nadal.

His drive for more major crowns is the reason he hasn't pulled out, making clear that if it was any tournament other than a Grand Slam he would have withdrawn.

But playing on could mean he faces a period on the sidelines after Melbourne to recover.

"I have talked a lot with my own medical team and also the medical team of Tennis Australia－Australian Open," he said.

"They all share the opinion that there is a slight, very slight, slim chance that I will make a significant damage that would take me out of the tour for whatever, some extended period of time.

"So, yes, there is always a risk that the injury will get worse, but they don't think it's going to be very significantly worse that it's going to jeopardize my entire season.

"So it will jeopardize, depending on how I go here, certain tournaments that are coming after Australian Open that I was maybe thinking to play."

Rafa rolls on

World No 2 Nadal beat old nemesis Fabio Fognini in a scrappy affair on Monday to reach his 43rd Grand Slam quarterfinal.

The Spaniard, bidding for a first title at Melbourne Park since 2009, has yet to drop a set and kept the record intact against the fiery Italian, winning 6-3, 6-4, 6-2 on Rod Laver Arena.

It is the 13th time Nadal has made the last eight of the Australian Open, moving to joint third on the all-time list behind only Federer (15) and John Newcombe (14).

The result keeps Nadal on track to meet Djokovic in the final.

"Happy to be in the quarterfinal, positive start," said Nadal, who is chasing his 21st major title and sole ownership of the men's Grand Slam record.

"When you play round of 16 against a great player like Fabio they are always fighting. You can't expect to go on court and not have some problems facing these types of players."

Nadal owned a 12-4 advantage over flamboyant Fognini dating back to 2013 coming into the clash and, despite some lower-back tightness, he looked sharp.

But it wasn't vintage tennis from the 34-year-old.

Earlier on Monday, Andrey Rublev ensured Grand Slam history will be made for Russia after setting up a quarterfinal against compatriot Daniil Medvedev.

With qualifier Aslan Karatsev already through to face Bulgaria's Grigor Dimitrov in the top half of the draw, it means there will be three Russian men in the last eight of a Slam for the first time since the Open era began in 1968.

Fourth seed Medvedev, who won the ATP Tour Finals in London last November, extended his win streak to 18 matches when he took just 89 minutes to blow away American Mackenzie McDonald 6-4, 6-2, 6-3.

World No 8 Rublev followed Medvedev onto Margaret Court Arena and was back in the locker room even quicker, as Norway's Casper Ruud retired injured with the scores at 6-2, 7-6 (3).

Looking ahead to facing Medvedev, Rublev said: "Last time he beat me in the quarters in the US Open. So now we're in the quarters in the Australian Open, so we'll see what's going to happen."

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