New men's world number one Daniil Medvedev says the only way to make him angry away from the tennis court is to knock on his hotel door at 6am seven days in a row.
Watching him play, it does not always appear that way.
In the heat of competition he can be irascible and cantankerous: the crowd's pantomime villain.
But the Russian can also be charismatic, spontaneous and wonderfully entertaining. By the end of a match he sometimes has the same crowd eating out of the palm of his hand.
On Monday, the 26-year-old became the first player other than Novak Djokovic, Rafael Nadal, Roger Federer and Andy Murray to climb to the top of the men's rankings since Andy Roddick 18 years ago.