On Wednesday, Novak Djokovic acknowledged that he was "pushed to the limit" by American Frances Tiafoe, a challenging player who took the world number one to four enduring sets at the Australian Open.
The Serbian top seed seemed in control following the first set, but lightning-quick Tiafoe, who arrived the quarter-finals two years ago, declined to go quietly.
He recovered to take a close second set before Djokovic dug deep to emerge a 6-3, 6-7 (3/7), 7-6 (7/2), 6-3 winner and continue his march towards a ninth Australian Open title.
"I was impressed with the way he was competing. He served very well, he fought hard. I thought we both played on a pretty high level. I mean, he pushed me to the very limit," said Djokovic.
"Just overall challenging conditions," he added.
"It was very hot, we had long exchanges. It was a tough match, and I'm really glad to overcome such a battle.
He will play another American, Taylor Fritz, in the third round after he battled past Reilly Opelka in five sets.
Djokovic, who dropped only six games in his opening round clash, had never played Tiafoe, but adjusted to the situation in no time.
On a sizzling day, with ice towels being used for the first time at the tournament, he immediately broke to race into a 3-0 lead, but some uncharacteristically poor serving permitted the energetic American back into the set.
It was a stumble that shook Djokovic who quickly broke again, pumping his fist in the air as he screamed at his box.
Neither player gave an inch in the tight second set, with Tiafoe's mix of power and unusual shots appearing to put Djokovic off his game as he bounced back on top in a tiebreak.
The Serb moved up a level in the third set, but so did Tiafoe as they traded blows until too many unforced errors made Djokovic took the set.
They went shot-for-shot in the fourth until a glorious backhand gave Djokovic the crucial break for 4-3, with a frustrated Tiafoe slapped with a warning for swearing then double-faulting to hand the top seed the match.
"He put me in a difficult spot. I had my chances early in the second set. If I broke him there, maybe the course of the match would be different.
"But again, he was holding his serve very well. I was not really using my breakpoint chances very well. At times I was too passive. Just wasn't feeling the ball today as well as I normally do," he further said.
Djokovic, pursuing an 18th Slam crown to close in on the 20 held by Roger Federer and Rafael Nadal, has succeeded a record eight times in Melbourne.
It was the scene of his maiden Grand Slam victory in 2008 and he is undefeated on the famous blue courts since his shock defeat by South Korea's Hyeon Chung in the 2018 round of 16.