Rivalries in sport are more often than not, just that - sporting rivalries.

Off the field of play, athletes are civil, courteous and even in some cases, mates.

However, the antagonism between 18-time Grand Slam winner Rafael Nadal and the so-called "bad boy of tennis", Nick Kyrgios, seems to go a little deeper.

The pair will face each other in the second round at Wimbledon on Thursday in the must-watch tie of the men's draw so far (and the third match on Centre Court).

But where did the rivalry come from, and why do these men really not get on?

A rivalry is born

Back in 2014 at Wimbledon, Nadal faced the fresh-faced and a little gangly 19-year-old Kyrgios in the last 16.

The Australian was already gaining his 'swaggering showman' reputation and had earned a wildcard spot at the tournament.

Ranked 144 in the world, Kyrgios stunned the Centre Court crowd hitting a staggering 37 aces, and became the first man outside the top 100 to beat a world number one at a Grand Slam since 1992.

The Aussie even pulled off the precocious 'hot-dog' lob, on his way to defeating Nadal 7-6 (7-5) 5-7 7-6 (7-5) 6-3.

Now, we may be making assumptions here, but that particular move - humiliating the great Nadal, and showing little regard for his then 14 Grand Slam titles - was perhaps what set off the chain reaction for their future rivalry.

After bursting onto the scene in that entertaining style, Kyrgios has faced Nadal five times since.

The wins have been shared equally between the two men but controversy just keeps on coming.

A Mexican stand-off

The latest was down in Acapulco in February 2019, where Nadal was incensed by Kyrgios' cheeky underarm serving.

After their second-round match at the Mexico Open, in which Kyrgios beat Nadal 3-6, 7-6, 7-6, Nadal accused Kyrgios of lacking respect.

"He is a player who has enormous talent," said Nadal having played his part in what appeared to be a frosty handshake with his opponent at the net.

"He could win grand slams and fight the top positions of the ranking, but there is a reason why he is where he is [then at a world ranking of No. 72]."

Ouch.

After the match, Kyrgios provided further evidence that he sees the Spaniard as his bitter adversary in a hostile Instagram post, showing the video of said frosty handshake.

"Don't doubt yourself, there are plenty of people who will do that for you. I can smell the blood when I play this dude", he wrote.

During a 'No Challenges Remaining' podcast interview published a few weeks later, Kyrgios also had this to say about the two-time Wimbledon winner:

"He's my polar opposite. Literally my polar opposite. And he's super salty.

"When he wins it's fine, he won't say anything bad, he'll credit the opponent - 'he competed well today, he's a great player' - but then as soon as I beat him, it's just like 'he has no respect for me, my fans and no respect to the game'. I'm like 'what are you talking about? I literally played this way that I beat you the other previous times and nothing changed'."

Polar opposites?

Nobody can argue the players have opposing temperaments - Nadal's measured, clinical and perfectionist ways seem the polar opposite to Kyrgios' unpredictability - not to mention his trademark short fuse.

Already in 2019, Kyrgios has had to hand over a few chunks of his earnings in the form of fines.

During the second round of the Italian Open, he was in the deciding set when he was given a game penalty, reportedly for swearing.

He then kicked a bottle, threw down his racquet, hurled a chair on the court, picked up his bag and walked off.

He was fined 20,000 euros (£17,461) for unsportsmanlike conduct and also lost his prize money.

Just a few weeks ago at Queens Club, he was once again fined £13,766 ($17,500) for unsportsmanlike conduct.

He accused a line judge of "rigging the game" and mocked the chair umpire for his headgear.

"It's a joke, man. It's a serious joke," he said. "Like your hat looks ridiculous, also. It's not even sunny."

Frenemies reunited

Kyrgios told the press he had been looking forward to this match-up since the draw was made last week.

"I was super happy that I saw him in my section. When you're a kid, you want to play the best players in the world on the best court in the world."

Nadal however, was a little less than pleased with the All England Club's draw policy, which takes into account player's performances on grass, as opposed to following the world rankings.

The world number two, who is seeded third behind Roger Federer (world number three), has said it "doesn't seem fair" as it means he has a tougher run to the final.

Asked in his post-match news conference on Tuesday, if he had a "good relationship" with Nadal, Kyrgios said: "Uhm, not sure that me and Rafa could go down to the Dog & Fox (a pub in Wimbledon Village) and have a beer together."

"I don't know him at all. I know him as a tennis player. I just don't - no, I don't know him very well."

Asked if he found it easier to talk to other players on tour, he added: "Yeah, 100%. That's just how it is. I get along with people, some people I don't get along with. We have a mutual respect and that's about it I think."

We will have to wait and see how "mutual" that is when they face each other on Thursday.