Daniil Medvedev has proved a difficult problem to solve for Stefanos Tsitsipas, the Greek tipped to reach a first Gland Slam final at this year's French Open and a player the Russian refers to as his "greatest rival".
In truth, the rivalry has been one-sided to this point.
Medvedev has won six of seven encounters, including the most recent in the semi-finals of the Australian Open. Tsitsipas' lone victory came en route to his triumph at the 2019 ATP Finals.
It started with a bang, an unseemly post-match spat forcing the chair umpire to intervene as the pair traded insults after their first meeting at the 2018 Miami Open, and their frosty relationship has persisted.
Medvedev had failed to win at Roland Garros in four previous visits but is slowly growing to appreciate the clay in Paris, where he could snatch the number one ranking from Novak Djokovic.
However, his affection for Tsitsipas remains limited -- and the feeling is mutual, with respect for each other's abilities but no great friendship.
"It seems to me that we respect each other as tennis players, but not too much on a personal level," Medvedev told Eurosport Russia after his last-16 win over Cristian Garin.
Tsitsipas once described Medvedev's playing style as "boring", and while a humbling defeat in Melbourne prompted a rethink, the two appear unlikely to break bread anytime soon.
"Our chemistry definitely isn't the best that you can find on the tour," Tsitsipas said following his win in London two years ago.
"It just happens with people, you can't just like everyone. It's not that I hate him. I guess -- yeah, as he said, we will not go to dinner together."
But there is much more than what Alexander Zverev calls their "weird relationship" that adds to the intrigue.
The straight-talking Medvedev, who excelled in physics and maths at school, keeps opponents guessing with his flat and low groundstrokes, while changing up the pace and angles, blending impenetrable defence with opportunistic offence.
"He tricks you. You know, he plays the game really smart," said Tsitsipas.
Disarmingly honest, Medvedev's propensity to speak his mind has at times landed him in hot water, most memorably at the 2019 US Open when he trolled the New York crowd after a sneaky middle-finger gesture.
Tsitsipas, in contrast, mixes aggression and patience, his style not all that dissimilar from his idol Roger Federer. The Greek is also more of a philosophical type, an aspiring photographer who produces his own podcast, and lists "anything creative" among his hobbies.
In short, Medvedev and Tsitsipas are vastly different characters -- ones that don't necessarily see eye to eye, but they can expect to see a lot more of one another in years to come as the faces of a new generation.