Defending Australian Open champion Novak Djokovic said a confrontation with a spectator was the spark he "needed" to hold off Alexei Popyrin and avoid a shock second-round exit.

Djokovic, chasing a record-extending 11th men's title in Melbourne, looked far from his best in the four-set win.

The Serb told one spectator to "come down and tell it to my face" during the fourth set after repeated heckling.

"Maybe that was needed for me to be shaken up a bit," said Djokovic, 36.

The world number one said he did not "want to be in those types of situations" but added the incident helped him to "start to find the kind of intensity on the court" that was required for him to see out a 6-3 4-6 7-6 (7-4) 6-3 victory.

The inspired Popyrin, ranked 43rd, entertained the partisan home crowd as he won the second set to level the match before losing a pivotal third set in a tie-break after failing to capitalise on four set points.

With the fourth set level at 2-2, Djokovic - who admitted he felt "flat emotionally" - said he engaged with the fan because his patience had run out after "a lot of things" were shouted at him during the match.

"There was a lot of things that were being told to me on the court, particularly from that corner, and the same side in the other corner.

"I was tolerating it for most of the match. At one point I had enough, and I asked him whether he wanted to come down and tell it to my face.

"He didn't have the courage to come down. That's what I was asking him. If you have courage, if you're such a tough guy, come down and tell it to my face, and let's have a discussion about it.

"He was apologising from far away. That's all it is."

Djokovic finds a way

Djokovic came through the longest first-round Grand Slam match of his career against teenage qualifier Dino Prizmic on Sunday, playing for four hours and one minute.

The early stages of his second match of the tournament suggested it would be a much quicker encounter for the Serb, who took 38 minutes to claim the opener after sealing the decisive break with a stunning cross-court winner to go 5-3 up before consolidating.

However, Popyrin - the world number 43 - proved to be a bigger threat than rankings suggested.

Having gone a break up in the fourth game, Popyrin missed the chance to serve out the second set, but he redeemed himself in front of his home fans with a sensational lob to break at 5-4 and level the match.

Djokovic survived four set points before racing through the tie-break to close out a dramatic third set and put himself in the driving seat.

And, after confrontating the heckler in the stands, the 24-time major winner thrived on the crowd's hostility to break for a 4-2 lead. Missing out on four match points, he claimed victory with the fifth as the clock struck midnight at Melbourne Park.

"I don't think I played at the highest level," Djokovic said. "I didn't do anything special - he was a better player for a set and a half. Things changed around and the momentum shifted in the tie-break."

Djokovic will face Tomas Martin Etcheverry next after the Argentine 30th seed, who conquered Andy Murray in straight sets on Monday, won 6-4 6-4 6-4 against Gael Monfils.

Sinner working on 'Baywatch' physique

Italian fourth seed Jannik Sinner stormed into the third round with a dominant win against Jesper de Jong.

The 22-year-old needed just 103 minutes to thrash the Dutch qualifier 6-2 6-2 6-2 under a closed roof on Margaret Court Arena on a rainy day.

Sinner, who will face Daniel Elahi Galan or Sebastian Baez in the next round, said he has been working in the gym on his off-days in Melbourne.

He joked he wanted a muscular physique like the actors in TV show Baywatch.

"Tomorrow I have a day off, trying to practise myself into the match rhythm. I [will do] also a little bit in the gym. Even if you can't see it, I'm skinny, but it's OK," Sinner said.

"I'm happy with my physicality at the moment. Of course it's a dream to have the 'Baywatch' physicality but it's OK."

Adrenaline rush carries Tsitsipas through tough test

Stefanos Tsitsipas said the adrenaline rush he felt was "insane" as he battled past Jordan Thompson in Wednesday's night session on Margaret Court Arena.

The Greek seventh seed required three hours and 36 minutes to overcome the Australian home hope 4-6 7-6 (8-6) 6-2 7-6 (7-4).

"You live for these kinds of matches, the intensity was there today. We were both able to peak in terms of our tennis at some point during the match," Tsitsipas added.

"The fact that I can go to sleep tonight and know I gave it all makes me extremely happy right now."

He will face Luca van Assche in the third round after Italian 25th seed Lorenzo Musetti suffered a surprise defeat at the hands of the French teenager, who fought back from 2-1 down to win 6-3 3-6 6-7 (5-7) 6-3 6-0.

American 17th seed Frances Tiafoe was another shock exit as he fell to a 6-4 6-4 7-6 (7-5) loss against Czech world number 75 Tomas Machac.

Tiafoe's American compatriots, 12th seed Taylor Fritz and 16th seed Ben Shelton claimed wins against French lucky loser Hugo Gaston and Australia's Christopher O'Connell, respectively.

Elsewhere, Russian fifth seed Andrey Rublev needed just 99 minutes to beat American Chris Eubanks 6-4 6-4 6-4, while Australian 10th seed Alex de Minaur swept aside Italy's Matteo Arnaldi 6-3 6-0 6-3.