Defending Australian Open champion Novak Djokovic held off Alexei Popyrin to avoid a shock second-round exit.
Djokovic, chasing a record-extending 11th men's title in Melbourne, looked far from his best as he won 6-3 4-6 7-6 (7-4) 6-3 on Rod Laver Arena.
Australian Popyrin entertained the partisan home crowd and clung on for three hours and 11 minutes before the world number one secured victory.
"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.
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.
Dominant on serve, Djokovic took 38 minutes to claim the opener, 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.
After a confrontation with a heckler in the stands, the 24-time major winner thrived on the crowd's hostility to break for a 4-2 lead and, having missed out on four match points, he claimed victory with the fifth as the clock struck midnight at Melbourne Park.
"[I'm] still trying to find form but in the early rounds you play players with nothing to lose," Djokovic said in his on-court interview.
"They come out to try and play their best tennis and I think both my first and second-round opponents were really great quality tennis players and I managed to find a way to win in four."
Sinner working on 'Baywatch' physique
Sinner has never gone further than the quarter-finals at the Australian Open
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 practice 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.
BBC