Federer won't stop picking up his racket just yet
There have been times, even in the recent past, when Roger Federer has made you wonder whether he might just go on forever. Who could have imagined that he would play one of the matches of his life, just a month before his 34th birthday, to deny Andy Murray at Wimbledon last summer?
Or that Novak Djokovic, having swept through 2015 in all-conquering style, would lose to the Swiss at last season's year-ending Barclays ATP World Tour Finals?
In the first two months of this year, however, the 17-time Grand Slam champion confronted his own mortality, at least in terms of his tennis life.
On the day after his loss to Djokovic in the semi-finals of the Australian Open, Federer was running a bath for his twin daughters when he turned and felt something click in his left knee. Within a week he was back home in Switzerland having surgery on a torn meniscus.
"There was a time of, say, 12 hours before and after the surgery and then going through the night and waking up when things were a bit fragile in my world," Federer admitted.
"I realised that you're like a passenger. You have to trust the people who are doing the surgery, that it's the right thing, that you'll be okay again. It was hard for me. I was very emotional. I was scared. Waking up from surgery, I was worried."
If Federer's recovery from the operation went as well as he could have expected - he was back competing in Monte Carlo within 10 weeks and would have returned four weeks earlier than that but for falling ill at the Miami Masters - it was not the end of his physical troubles.
A back problem kept him out of the recent French Open, which ended his record run of 65 consecutive appearances in Grand Slams. The last he had missed was the 1999 US Open.
You learn never to write off Federer, but as he prepares to contest his 18th successive Wimbledon it is impossible to wonder how long he has left in top-level competition.
He returned to fitness this month to play two grass-court tournaments, but the symbolism in his defeats in Stuttgart and Halle to two of the game's most exciting youngsters, 22-year-old Dominic Thiem and 19-year-old Alexander Zverev, was inescapable.
Federer, who will be 35 in August, admitted he had cherished the extra time at home but insisted that he still enjoyed getting back on the road.
"I think it works both ways, for me anyway," Federer said. "Whenever you leave your home, especially if you've been there for a while, you're happy to be in the car with a trunk full of luggage. You might be travelling two hours or 20 hours, but you're excited to go.
"That has always been there and it's the same the other way. The moment you come back home it's nice to drive up to your house.
"As a family man and just as a tennis player, you love those times when you're in one place for a longer period of time, even more so when it's at home."
Had this year's experience not made him think that he would like to spend more quality time at home on a more permanent basis? After all, Federer and his wife Mirka have two sets of twins - six-year-old Myla and Charlene and two-year-old Leo and Lenny.
"That will come one day regardless of whether I want it or not," he smiled. "When my tennis is over I can choose how much I want to have of that. It doesn't need to be right now. That's why the thought doesn't cross my mind, because I'm happy to play and travel."
Was the knee injury - and the first surgery he has ever had to undergo as a professional -a reminder that his career could finish at any stage?
"Yes, but I knew that before I got injured," Federer said. "I always said that waking up and finding that the motivation is not there any more is something that could happen to any player. I have a hard time understanding how that can happen, but it could. Or you could get injured.
"Also, if something happens to your family, your parents or your friends, that can really rock your world. That's also something that can put everything in perspective and you retire because of that."
Does he ever wonder what he might be doing in 10 years' time? "I would like to spend a lot of time with my wife and my kids," he said. "I would probably have more time with my wife on a daily basis. Even though I spend every day of my life with her, it's all around taking care of the kids.
"That's all going to be easier as we move on. Bigger kids, bigger problems, I get that. But you're not going to see them as much. We already see it now when we send them to tennis camps or tennis lessons. The next thing you know you don't see them for a day. It's good for them but not great for us. That's only going to happen more with school coming in, so we'll have more time for ourselves."
For the time being, Federer insists that retirement is not on his radar. "I've tried to put zero thought into it, because I feel like it needs to be a natural decision," he said.
It helps that Federer loves the life of a player, even if there are aspects he could do without. Like what? "Getting cameras in my face, especially with the telephones now," he said. "You might think that's normal. I understand it, but it's constant.
"Do I enjoy being the centre of attention? Yes, sometimes, but there are things you don't like. Do I like being stuck in traffic going to the airport with the kids screaming? I'm not sure. But it's part of the tennis life, 99 per cent of which is nice.
"I just think you have to be clever in your scheduling to make it fun for yourself. I calculate how many jet lags I want go through and how many I want to put my kids through. Being based in Dubai as a tennis camp is actually good thing for me.
"Then you have to make it fun with the hotels. I drive myself at 40 per cent of the events. I drive myself in Australia, in Dubai, Indian Wells, Miami, Cincinnati. I love to drive.
"Or when we have a driver I have the same driver I've had for the last 10 years, so basically we're friends now. So you can make it fun for yourself on the tour. You just have to know how to do it."
When the day does come to put his rackets away, might he still have a future in tennis? "I would hope so though I don't know in what field," Federer said.
"I haven't thought about it a whole lot, but I've always loved tennis. Tennis has given me all and more. Of course I'd like to stay involved in some ways. Maybe just on a very simple level, helping kids in Switzerland. Who knows?"