If you run into Roger Federer on your way to board a plane, do not be surprised if he looks you up and down, or mostly down. That’s because Mr. Federer has something of a sneaker fixation.

The Swiss tennis champion is not an “obsessive,” he said (slightly hard to believe about someone who has won 20 Grand Slam trophies), but still he is “always looking at other people’s feet in airports.”

Earlier this month, when Mr. Federer was in London to play in the ATP Finals, he said in an interview that he owns at least 250 pairs of sneakers. Then, pulling on a burnt orange pair with terra cotta laces and distinctive hollow little pods that ran the length of the soles, he acknowledged that that was probably a conservative estimate, given all the shoes he has from past matches.

So, realistically, double? Possibly, he said with a grin.

Mr. Federer, 38, would go on to lose the ATP semifinals, to the eventual winner, Stefanos Tsitsipas, a 21-year-old born the same year Mr. Federer played his first professional tennis game.