Watch: Roger Federer gets serenaded by school kids in Lucerne

While Roger Federer may not be visible on the tennis courts, he has kept his hands full by building playgrounds for schools through his foundation.

The Roger Federer Foundation has financed and co-designed 31 so far in the Swiss maestro's home country and is planning to build 65 more by 2025, according to an article by sports editor Simon Graf on Tages Anzeiger.

Graf posted on Twitter about a touching moment shared between Federer and the school kids in Emmen Dorf, Lucerne, where the 20-time Grand Slam champion recently opened one of his Natural School Playgrounds projects.

"A song for @rogerfederer from the schoolkids im Emmen, Lucerne, after the opening of the new playground, funded by @rogerfedererfdn. Do you recognize the song with lyrics in Swiss German?" Graf tweeted.

The kids sang 'With a Little Help From My Friends' in Swiss German. Federer, who was seen attentively listening and clapping along with the children, then spoke to the kids thereafter.

"Take care of each other and the playground," said Federer in Graf's article.

In the same article, Federer also offered advice as to what young, aspiring tennis players should focus on.

"In general, activities that promote coordination are good, such as climbing, jumping, doing somersaults. The understanding of the body has been somewhat lost in many children," said Federer.

The eight-time Wimbledon champion also warned against pushing the child too much at an early age.

"And if a child has tennis training three times at the age of six, there is a great danger that it will say: Now I don't want to play tennis anymore!" said Federer.

Roger Federer Foundation CEO Janine Handel: Roger is not a rock star, he is approachable

Roger Federer spends time with the kids during the inauguration (Photo courtesy of the Roger Federer Foundation Twitter account)

Roger Federer is actively engaged in his foundation. Last April, Federer visited Malawi in Africa for his School Readiness Programme, an initiative focusing on providing access to quality pre-primary schooling and ensuring that children, their families and their schools are equipped to receive, support and provide early childhood education.

Roger Federer Foundation chief executive officer Janine Handel, in a separate interview with Simon Graf, said that the reception towards Federer during these visits has always been that of admiration and respect. Despite his stature, Federer is approachable and is "not a rock star," according to Handel.

"A rock star? No, Roger is not a rock star. He is approachable, a rock star is not. And when we visit a kindergarten in Africa, nobody knows who he is. It is always amazing to watch him during his visits. He plays along straight away, tackles things and is integrated very quickly," said Handel.

"With Roger, the children immediately lose their inhibitions... Because these children are not used to strangers and certainly not white people. Roger can open people at the touch of a button: children, but also adults. That is an impressive experience for me. The children are usually afraid of strangers, and I've even burst into tears. But Roger knows exactly how to gain the trust of the children and the kindergarten teachers," Handel added.

Handlel, in fact, believes that the term "rock star" is inappropriate for the former World No. 1.

"You feel that he likes you and respects you. He treats everyone as equals. People take him seriously. Therefore, for me, there is no more inappropriate term for Roger as a rock star," said Handel.