Serena Williams' coach at centre of fresh controversy at Australian Open

Stefanos Tsitsipas was hit with a rare code violation for coaching on Wednesday night in his extraordinary quarter-final win over Rafael Nadal.

Tsitsipas became just the second man to ever beat Nadal at a grand slam after losing the first two sets, staging an incredible comeback to win 3-6, 2-6, 7-6, 6-4, 7-5.

'NOT GOOD ENOUGH': Ugly truth in Ash Barty's Australian Open exit

'PATHETIC DECISION': Tennis fans erupt over Dylan Alcott 'disgrace'

However the match wasn't without a touch of controversy after Tsitsipas was hit with a code violation in the fourth set when the chair umpire spotted him receiving some coaching from his box.

While Tsitsipas' father and coach Apostolos was seen sitting in the box, Serena Williams' coach Patrick Mouratoglou was also spotted.

Mouratoglou has been working with Tsitsipas for a number of years, as well as coaching Williams.

As Jim Courier pointed out in commentary for Channel Nine, Mouratoglou was at the centre of the most infamous coaching violation in tennis history.

"The most famous coaching violation in the world was Serena Williams with Patrick Mouratoglou against Naomi Osaka at the US Open final a couple of years back… just a reminder," Courier said.

Todd Woodbridge added: "He (Tsitsipas) was told they were warned a little while ago.

"They were told and the umpire has followed."

Mouratoglou was in Serena's courtside box during the 2019 US Open final when she was pinged for a coaching violation.

It was one of three code violations that Serena received during her extraordinary meltdown, which largely overshadowed Osaka's maiden grand slam triumph.

As Courier pointed out, it wasn't clear whether it was Mouratoglou who was pinged on Wednesday night.

"There's no way to know if it was Patrick or his dad (Apostolos), they're both coaches," Courier said.

"It's also worth pointing out that a player gets penalised for something they are not necessarily doing in that environment.

"That's how it works. If a coach says something they shouldn't say, it's the player who gets the penalty."

Tsitsipas shocks tennis world with Nadal comeback

After looking completely outmatched during the first two sets, Tsitsipas hunkered down to pinch the third set in a tiebreak, then went on to claim the match of the tournament to set up a last-four clash with Daniil Medvedev.

"It's an unbelievable feeling to be able to fight on such a level," Tsitsipas said after the match.

"I don't know what happened after the third set, I flew like a little bird - everything was working for me."

It marked just the second time in Nadal's 19-year grand slam career that he'd lost a match after being two sets up, the only other occasion coming against Fabio Fognini at the 2015 US Open.

He'd lost one other Masters series match against Roger Federer the same way.

An exceptional standard of tennis by Nadal in the first two sets gave little indication of the drama that was to unfold.

The 34-year-old looked almost untouchable, striking winner after winner, hitting just 12 unforced errors and not facing a break point.

Nadal then lost just a solitary point in his six service games in the third set but Tsitsipas refused to cede on his own delivery and took it to a tiebreaker.

A handful of Nadal mistakes then saw the match pivot.

Extracting two Nadal errors off smashes and a shanked forehand to bring up three set points, Tsitsipas took advantage of the second to bring the match alive and never looked back.

with AAP

Click here to sign up to our newsletter for all the latest and breaking stories from Australia and around the world.