Nick Kyrgios stunned the tennis world with his freakish fightback against Rafael Nadal overnight β€” but it's the off-court drama that is now raising eyebrows around the globe.

Nadal served up a verbal lashing to Kyrgios after the defeat, saying he "lacks respect" after the Australian's stunning three-set win over the world No 2 at the Mexico Open in Acapulco.

Nadal's salty comments to Kyrgios came after the pair exchanged an ice-cold handshake at the end of their match after Kyrgios twice managed to get under the Spaniard's skin.

Kyrgios sensationally attempted an underarm serve during the match because Nadal had retreated miles behind the baseline to return Kyrgios' booming serves.

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The pair also clashed over a time-wasting accusation in the tense third set tiebreak where Kyrgios complained to the chair umpire because of Nadal's attempt to slow down and disrupt his natural service rhythm.

Nadal then proceeded to declare Kyrgios "lacks respect" for his opponent, himself and the crowd in his post-match press conference.

Kyrgios responded by saying he won't be listening to Nadal's criticism and said Nadal "doesn't know anything about me".

Those terse words are only just the beginning.

Kyrgios took to social media after his stunning win, where he appeared to pour more petrol on the simmering tension.

He posted a video of match point and his cold handshake with Nadal to his Instagram page and appeared to make a reference to Nadal's previous links to the Operation Puerto doping scandal that rocked Spanish sport a few years ago.

His caption to the video included a needle, ghost and robot emojis.

"Don't doubt yourself, there are plenty of people who will do that for you," Kyrgios wrote.

"I can smell the blood when I play this dude."

The Operation Puerto doping scandal is still yet to be concluded β€” almost 13 years after police first launched an investigation into an alleged doping network in Spain.

Kyrgios' claim that he could "smell blood" could be interpreted as a reference to the 211 bags of blood WADA is in possession of following a seizure operation.

The World Anti-Doping Agency (WADA) last year filed an appeal to the Constitutional Court of Spain in a bid to release the names of the athletes allegedly involved in the network.

Nadal has previously strongly denied claims linking him to the doping plot and there is no suggestion he has done anything untoward.

WADA is still in possession of more than 200 blood bags seized by police in a sting operation, but have never been able to reveal the names of the athletes alleged to be involved.

Eufemiano Fuentes, the doctor at the centre of Operacion Puerto, admitted in 2016 that some "major names" could be uncovered in the investigation.

Nadal has previously vigorously defended his reputation against the unproven accusations and also launched defamation action against the first person to publicly accuse him of any involvement in the Operation Puerto investigation, a former French government official.

Rafael Nadal claimed Nick Kyrgios lacked respect during his match against the Australian. Photo / Getty
Rafael Nadal claimed Nick Kyrgios lacked respect during his match against the Australian. Photo / Getty

He has also publicly called on the International Tennis Federation previously to release his drug tests to prove he is not a doping cheat.

"There has to be complete transparency in sport," Nadal said in 2016.

"When things are not done the right way, they should be public. If not, we're going to be speculating about unnecessary things, which is going to damage the image of the sport and sportsmen."

The blood bags seized by police in 2013 were ordered by a Spanish court to be destroyed before the decision was overturned on appeal by WADA.

Kyrgios' Instagram post wasn't his only shot at Nadal on social media.

He commented on Facebook that his underarm serve was a perfectly legitimate tactic while Nadal chose to return from so far behind the baseline.

"That's the only reason for doing it," he said in response to a fan's praise for taking such a bold approach with his serve.

"I mean isn't the idea to serve where the person can't get the ball? Try to get an ace?"

Nick Kyrgios' Facebook post. Photo / Facebook
Nick Kyrgios' Facebook post. Photo / Facebook

Kyrgios saved three match points against Nadal to pull-off arguably his best result in almost two years.

But what should have been a highlight after a difficult 18 months turned sour in the media conference when Nadal took aim at the temperamental world No 72.

"He's a player who has enormous talent, could be winning grand slams or fighting for the No 1 ranking," Nadal said in Spanish.

"He lacks respect for the crowd, his opponent and towards himself."

He went on to say: "I don't think he's a bad guy, but he lacks a little respect for the public and the rival."

There were several flashpoints in the match which might have upset Nadal. During the first set, Kyrgios appeared not be giving a full effort on occasions and even flirted with pulling out of the match.

"I feel sick ... if I forfeit, the media will blow it up ... I'll be booed off the court," he said during the set.

"I'm gonna try to play a couple of more games."

Nick Kyrgios of Australia celebrates after winning his match against Rafael Nadal. Photo / Getty
Nick Kyrgios of Australia celebrates after winning his match against Rafael Nadal. Photo / Getty

Kyrgios received a warning for swearing and telling the crowd to "shut up" during the second set.

Kyrgios hit back at Nadal when it was his turn to front up to reporters.

"I'm different, Rafa's different. He can focus on what he needs to do. He doesn't know the journey I have been on, he doesn't know anything about me," said Kyrgios, who famously stunned Nadal as a teenager at Wimbledon in 2014.

"I am not going to listen at all. That's the way I play. The way he plays is very slow between points.

"I've got my game, he's got his game. People are different, that's the sport."

Nadal appeared to be upset in the tense third set tiebreak when Kyrgios complained to the chair umpire about the 17-time grand slam champion taking too long in between points when Kyrgios was serving.

Yet the quality of play and perseverance couldn't be questioned. He saved nine break points throughout the match including coming back from 0-40 positions in the final two sets.

Kyrgios also saved three match points, including one with a lunging volley that hit the net cord and just landed in.

It was Kyrgios' best win since the Cincinnati Masters in August 2017, when he also took down Nadal in straight sets in a quarter-final.

There might be more controversy ahead as Kyrgios will next play arch-rival Stan Wawrinka on Saturday.