Roger Federer is a 19-time Grand Slam winner.

Over the years, he has won everything in terms of major honours, and he's more than familiar with sweeping up trophies at the smaller tournaments as well.

However, despite this and despite his massive success over the years, he does actually have regrets.

Not long ago he mentioned that he regrets not pushing Rafael Nadal all the way for the number one spot this season, but he's now revealed what's the biggest regret in his entire career.

Maybe somewhat surprisingly, it isn't what you'd think.

Federer has expressed his regret at not being able to watch FC Basel matches as much as he’d like because of his career in tennis.

The Basel-born 36-year-old has been pictured wearing the jersey of his boyhood club, as well as visiting St. Jakob-Park, their home stadium, and was given the honour of presenting the FC Basel players their championship trophy after they won the league in 2016.

But, a career and indeed a life dedicated to tennis has meant that Federer has had very little chance to follow the fortunes of the 20-time Swiss Super League champions as much as he’d like, but he insists that once he retires that will most likely change.

“I’d like to attend more FC Basel games and this kind of events,” he said.

“But this is what you have to do (make sacrifices) if you have a tennis career.

“But, I know that when I will stop playing tennis it will all be different. I have an apartment in the centre of the city.”

The 19-time Grand Slam champion also stated that he missed spending time in the city of his birth and he revealed that his children are well aware of their Basel-based heritage, as they still have family who live there.

“They (his children) know I had my childhood in Basel because they have grandparents here and aunt and cousins.”

The world number two also mentioned that due to his wife Mirka’s family home being in Thurgau, in the North-East of the country, he didn’t get to spend as much time with his own family as he would like either.

“I miss not being in Basel, and I will probably always have that feeling. I would like to go see my sister Diana, my parents too.

“This is my family. They realise it. They also know I am an FC Basel fan. They also ask me if Basel won.”

FC Basel’s recent success has been intertwined with Federer’s as the Swiss club have dominated domestically since 2003, around the time their most famous supporter began to stamp his own mark at the summit of the world of tennis.

Perhaps some time soon, the eight-time Wimbledon champion will be able to experience much more of FC Basel’s prosperity for himself. Although tennis fans will be hoping, not too soon.