Andy Murray stunned Italian 13th seed Matteo Berrettini on Tuesday, saving a match point to prevail over five gruelling sets and book a spot in the Australian Open second round.

The three-time Grand Slam champion has battled major hip and abdominal injuries for half a decade but declared himself free of pain and fired up before the tournament.

His renowned grit was on full display in toppling the Italian 6-3, 6-3, 4-6, 6-7 (7/9), 7-6 (10/6) in 4hrs 49mins under a closed roof on Rod Laver Arena due to extreme heat.

It was a huge win for the 35-year-old Briton, now ranked 66, to knock out last year's semi-finalist for a landmark 50th match-win at the season-opening Grand Slam.

"I'll be feeling this this evening and tomorrow, but right now I'm unbelievably happy and proud of myself," said Murray, a five-time runner-up at Melbourne Park.

"I've put a lot of work into the last few months with my team to give me this opportunity to perform on stadiums like this, in matches like this and against players like Matteo and it paid off tonight."

It was Murray's first victory over a top 20 player at a Grand Slam since defeating ninth-ranked Kei Nishikori in the quarter-finals of the 2017 French Open.

Victory also brought up his 50th Australian Open match-win, becoming only the fifth man in the Open Era to achieve the feat alongside Roger Federer, Novak Djokovic, Rafael Nadal and Stefan Edberg.

"I think some of the tennis at the end was really good, it felt like that playing," he said.

"He's a brilliant competitor, one of the best on tour, he always fights to the end and I did well to get through."

Coached by Ivan Lendl, who won the Melbourne Park title in 1989 and 1990, Murray broke once in the first set and twice in the second as the Italian struggled on his second serve while making a slew of unforced errors.

But Berrettini rallied in the third set with a break to move 3-2 clear and helped by six aces took it to a fourth set.

Nothing separated them and it went to a seesawing tiebreak that the Italian won on his third set point when Murray sent a forehand long.

Berrettini earned a match point 5-4 in the fifth, but netted an easy half-volley to waste the opportunity and it again went to a tiebreak where an early break for Murray gave him the edge and he clung on.