Five years ago, I asked Paganini if Federer, then 31, could play on until 2016. He said he saw no reason to doubt it based on Federer’s playing style, talents and enduring enthusiasm.

How about 2020?

“I think only Rog will know when it’s the moment that he’ll want to say perhaps this is enough,” Paganini said. “Rog does have the biological age of 36 but for me, he has an athletic age that is younger than that and yet he has the maturity of someone well over 40. So it’s quite a balance. And because of that it’s very difficult to say or predict. It’s the man who makes the decision, not just the athlete, unless there’s a serious injury that leaves you no choice.”

The two met when Federer was 14 and a new arrival at the Swiss national training center in Ecublens near Lausanne. Federer, the youngest boarder there, was still working through his talent and his temper. When he eventually put together his own team in 2000, he asked Paganini to join him.

“He’s made fitness workouts so enjoyable, if they ever can be,” Federer said. “I just follow his beat. Whatever he tells me I’ll do it because I trust him. People ask me, do you still do your physical tests and stuff? I don’t have to do any tests because I work with Pierre and he knows and sees if I’m moving well or not; if I’m slow or fast; all these things. He’s had a huge part of this success, and I’m happy I called him way back when.”

Paganini said his methods of training Federer have changed with the years. They used to play other sports like basketball in Federer’s youth to add variety but now focus on activities that directly correspond to tennis and, according to Paganini, they emphasize complex drills that mimic the multipronged challenges of the sport.

“You have to be strong, fast, coordinated and have endurance in tennis and you have to do drills for that,” Paganini said. “But you also should never forget you have to use this on a tennis court; not on the road or in the pool. So you always have to create a link between the speed and the athletic way it’s used on the court. Nine times out of 10 on the court, the speed is in the first three steps and then you’re playing the tennis ball. So you have to train to be particularly strong in the first three steps.”