Aussie tennis legend Lleyton Hewitt appeared to break the rules during Alex De Minaur's dramatic four-hour five-set defeat to Andrey Rublev at the Australian Open on Sunday evening.

The local hope fought back from losing the first set to snag a two-sets-to-one lead at Rod Laver Arena on Sunday and appeared destined to kick on as a frustrated and emotional Rublev waged a verbal war on himself.

But a brilliant Rublev came home with a wet sail to level proceedings then, as de Minaur crumbled, put the foot down in the fifth set to win 6-4 6-7 (5-7) 6-7 (4-7) 6-3 6-0.

However, with Rublev leading 4-0 in the fifth set it looked as though the Russian was starting to cramp up.

Rublev was pulling up short on the baseline and was stretching his calf between points.

Hewitt spotted this and tried to get De Minaur's attention so he could take advantage of the situation. 

Coaches are permitted to courtside coach under new rules, but only when they are at the same end as the player.

Hewitt, who was situated at the other end of the court, appeared to gesture at De Minaur to make the Russian run more - however he stopped short when he noticed himself on the big screen at Rod Laver Arena.

Aussie Tennis great Todd Woodbridge acknowledged how frustrating it must've been for Hewitt.

'He was down the other end at the time and he's wanting to say 'Did you see it, did you see it?',' Woodbridge told Nine's Morning Serve.

'And he couldn't sort of get the message across because he only needed to get the ball going corner to corner and kind of get Rublev moving, but that was what was amazing with Rublev. 

'He'd go bang on the serve, crank a winner, or get the point where he could just stand in the centre and that's hard to do and he just kept his composure.

'Unbelievable when he was this close (gesturing with his fingers) to being not able to finish the match.'

The loss sparked heartbreaking scenes post-match where a shattered de Minaur was spotted in the players' gym being consoled and embraced by his English girlfriend, fellow tennis player Katie Boulter.

Fellow Australian Nick Kyrgios applauded his efforts in a match played at an 'insane' level of athleticism, while American legend John McEnroe reckoned de Minaur had only been beaten by a man playing 'one of the greatest fifth sets I've ever seen in a grand slam'.

'It was one of the all-time great efforts by Rublev, who's taken it up to the next level,' said McEnroe on Eurosport.