Novak Djokovic came through his first major test at the Australian Open against American powerhouse Frances Tiafoe.
The world number one, bidding for an unprecedented ninth Melbourne title, had dropped only six games as he flattened Jeremy Chardy in round one.
But a fired-up Tiafoe proved a tougher proposition, taking a second-set tie-break and going toe-to-toe with Djokovic in the third, before the Serbian battled through 6-3 6-7 (3-7) 7-6 (7-2) 6-3.
Djokovic had two points for 4-0 in the third but, seemingly struggling with the 30C temperature, he allowed Tiafoe to draw level.
Tiafoe saved two set points at 4-5, but with the 23-year-old from Maryland now also feeling the heat Djokovic pulled out a near-perfect tie-break.
Some lung-busting rallies in the fourth left both players gasping for breath and it was Tiafoe who lost his way down the finishing straight.
A time violation ultimately cost him his serve to trail 4-3 and a double-fault on match point gave Djokovic victory in three and a half hours.
"It was a very tough match, it was very warm with the sun on the court and a lot of long rallies," Djokovic, who fired down 26 aces, said on court afterwards.
"I want to give Frances a hand for a great match. If I had to choose I wouldn't like to have these matches early in the week, but the matches are only going to get tougher.
"I know how to handle these circumstances, but I was lucky to get through the third set, it was anyone’s game. It was high-quality tennis."
Former champion Stan Wawrinka missed three match points in a dramatic five-set loss to Hungary's Marton Fucsovics.
The 2014 winner, seeded 17, was two sets down but fought back to level and then broke back when Fucsovics served for the match in the decider.
He looked set to complete the comeback when he led 9-6 in the first-to-10 points deciding tie-break, only for Fucsovics to reel off five in a row to prevail 7-5 6-1 4-6 2-6 7-6 (9-7).
The Nick Kyrgios show was in full effect as the colourful Australian won a crazy five-setter after saving two match points against Ugo Humbert.
Yet a lot of the usual controversy which tends to follow the temperamental 25-year-old around was seemingly not all of his own doing this time.
Already a set and a smashed racket down, Kyrgios was serving to level the match at 30-30 and fired down what he thought were two clean aces, only for a let to be called both times.
He certainly appeared to have a point as he claimed the ball had not clipped the net and implored umpire Marijana Veljovic to turn the let technology off.
"It's ruining the game. Do you not understand that?" he claimed, pointing out that a Humbert serve had also been incorrectly called.
Having taken the set, and celebrated wildly, Kyrgios came up with a scarcely believable behind-the-back lob at the start of the third.
He lost that point, and was deducted another for a second code violation as Humbert, 22, relentlessly served out to lead 2-1.
The French 29th seed was on the brink of victory until Kyrgios came alive at the end of the fourth set, saving the two match points before roaring through the tie-break to level the match again.
Kyrgios looked on the verge of full meltdown again after another row about let calls early in the decider, but from break point down he promptly won 11 of the next 12 points to lead 3-1.
The fired-up world number 47 dropped just two further points on serve as he somehow wrapped up a 5-7 6-4 3-6 7-6 (7-2) 6-4 victory.
Afterwards he turned the air as blue as the court in his post-match interview, saying: "Honestly if you were in my head I was just thinking about all the s*** I was going to cop if I lost that match. It was one of the craziest matches I've ever played."
Next Kyrgios will face US Open champion Dominic Thiem, who brushed aside Germany’s Dominik Koepfer 6-4 6-0 6-2, while sixth seed Alexander Zverev eased through 7-5 6-4 6-3 against American qualifier Maxime Cressy.
Denis Shapovalov, the 11th seed, beat Australia’s Bernard Tomic 6-1 6-3 6-2 to set up an enticing clash with fellow Canadian Felix Auger-Aliassime, while Diego Schwartzman dropped only five games in beating lucky loser Alexandre Muller 6-2 6-0 6-3.