Undivided devotion is usually associated with team sports. Travelling hundreds of miles to watch your heroes, supporting them through thick and thin. Naming a kid after a favourite player. Getting a tattoo.
Yet Roger Federer and Serena Williams - two superstars who retired this month - have attracted devotion like few others.
And it felt fitting that two people born 49 days apart, who enjoyed era-defining parallel careers, called it a day within weeks of each other.
"These two players are irreplaceable. I don't think there is any question about that," said John McEnroe, another legendary tennis figure.