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 slightly 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 kick-started their future rivalry.
Kyrgios has now faced Nadal six times, with the wins shared equally between the two men, but controversy is never far away.
A Mexican stand-off
The latest encounter came in a second-round match at the Mexico Open in Acapulco in February, when Nadal was incensed by Kyrgios' underarm serving.
Kyrgios beat Nadal 3-6 7-6 (7-2) 7-6 (8-6), and afterwards Nadal accused the Australian of lacking respect.
"He is a player who has enormous talent," said Nadal after what appeared to be a frosty handshake 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 72]."
Kyrgios responded with a hostile Instagram post, showing the video of the 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'."
Nobody can argue that the players have different temperaments - Nadal's measured, clinical and perfectionist ways seem a world away from Kyrgios' unpredictability.
Kyrgios has already had to hand over a few chunks of his 2019 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 to 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 Queen's Club, he was once again fined £13,766 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."
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," he said. "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 seeding policy, which takes into account player's performances on grass, rather than following the world rankings.
The world number two, who is seeded third behind Roger Federer, 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, whether 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 whether 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.