Former world number one Andy Murray has gallantly fought back from two sets down to defeat Thanasi Kokkinakis at the Australian Open, with the match lasting five hours and 45 minutes.

Murray seemed headed for a second-round exit from the tournament after Australian hope Kokkinakis served for the match in the third set, but the 35-year-old Scot climbed off the canvas to win 4-6, 6-7 (4/7), 7-6 (7/5), 6-3, 7-5.

With the clock on Margaret Court Arena reading 4:05am AEDT, Murray and Kokkinakis embraced at the net, as the spectators who had stuck around to watch the climax of the match roared with delight.

It is the second-latest finish for a match in Australian Open history, pipped only by Lleyton Hewitt's five-set epic against Marcos Baghdatis in 2008, which concluded at 4:34am AEDT.

What made Murray's win even more remarkable was the fact he was coming off a five-set win over Italian 13th seed Matteo Berrettini in the first round, that match lasting four hours and 49 minutes.

After the loss, a clearly exasperated Kokkinakis tweeted: "This f***ing sport man", alongside a pensive, remorseful emoji.

Kokkinakis appeared to have the victory wrapped up when he served for the match at 5-3 in the third set.

However, three-time major champion Murray broke the Australian's serve and went on to claim the set in a tie-break, before levelling the match by winning the fourth.

As both players fought off fatigue, the quality of tennis did not diminish in the final set, with Kokkinakis and Murray each reeling off superb backhand winners.

Kokkinakis's serve was also finding its range, with the 26-year-old sending down a pair of aces on his way to taking a 3-2 lead as the match crept past the five-hour mark.

The rallies had not lost any of their intensity as Murray held again, to level at 3-3, before he seemed set to gain the crucial service break in the following game.

Kokkinakis found himself 0-40 down to present Murray with three break points but he saved each, with a brilliant crosscourt forehand bringing up deuce.

He was not out of the woods, though, as Murray was to enjoy a fourth break point, but Kokkinakis saved once more and went on to hold serve.

The set wore on, with neither player willing to give up an inch, while Kokkinakis turned to his serve again when he fired off three aces in the ninth game as he looked to preserve energy by keeping the points short.

In the 11th game, the turning point finally came. With Kokkinakis serving, Murray had four break points for a 6-5 lead, which would give him the opportunity to serve for the match.

He converted on the fourth with a forehand winner, before pumping his fists as he turned to face his support team in the crowd.

Murray made no mistake serving out the match, with a crisp backhand return icing his remarkable win.

The five-time Australian Open finalist will play Roberto Bautista Agut in the third round.

Heading into the match, fatigue was a factor for both players, with Kokkinakis having to contend with his first-round encounter against Italy's Fabio Fognini being played across two night because of rain delays.

That left him with only 24 hours to prepare for the match with Murray.

Despite the result, it was a performance from which Kokkinakis could take great pride, highlighted by his staggering ace tally of 37.

Kokkinakis makes bright start

After cool weather conditions and a vocal crowd greeted the two combatants when they arrived on court after 10pm AEDT, Kokkinakis was in danger of dropping his opening service game, with Murray earning three break points.

However, Kokkinakis — who was hoping to join countrymen Alex de Minaur and Alexei Popyrin in the third round — held and, three games later, had his opponent's serve under the pump.

He broke Murray and consolidated the break to lead 4-2, before going on to comfortably win the set.

The second set was an arm wrestle, with neither player coughing up a break point until the ninth game.

It was Murray who found himself under pressure and after going long with a backhand return on the first of three break points, he allowed Kokkinakis to serve for the second set at 5-4.

Kokkinakis looked certain to pounce when he had two set points at 40-15, but Murray hustled his way back into the game and broke to level at 5-5.

Each won their next service game to force a tie-breaker, which Kokkinakis claimed with an ace down the middle to establish a two-sets-to-love lead.

After securing a break, to lead 2-0 in the third set, Kokkanakis stumbled when he was broken in the very next game, during which he remonstrated with the chair umpire after being handed a time code violation.

He then attracted a code violation and warning for tossing his racquet on the court at the end of the game, as Murray revved up the crowd, knowing he was back in the set.

Kokkinakis recovered from the meltdown and in the sixth game he broke Murray's serve for a 4-2 lead.

He served for the match in the ninth game, only for Murray to break back ahead of winning the third-set tie-break.