Roger Federer issued a fun challenge to fellow players at fans on social media to keep themselves occupied during the ongoing shutdown due to the coronavirus pandemic.

The Swiss shared a one-minute video of a solo drill to try at home, where he rapidly volleyed a tennis ball against the wall. A lesson on volleying from one of the greats, what could be better? A funky hat and feedback from the master himself!

Federer tagged a bunch of other athletes to try it and asked everyone to share their videos doing the drill. He would watch the fan videos and give some tips. The only catch? You had to wear a hat while doing it. “Choose your hat wisely,” Federer wrote, flaunting a white wide-brimmed in the video.

Instantly, the fun fitness challenge blew up all over social media and the 38-year-old kept it going with witty replies to fans who shared the videos with hats on. “Don’t lean back, strong in the wrist,” Federer wrote to one person.

Among the celebrities who took up the challenge were Australian Open champion Sofia Kenin, Coco Gauff, and Martina Navratilova, footballer Toni Kroos, Hollywood star Hugh Jackman and skiers Lindsey Vonn and Mikaela Shiffrin.

Long-time rival Novk Djokovic rose to the challenge as well, volleying the ball as good as the Swiss. The Serb didn’t wear a hat but the banter between the two made the exchange amusing nonetheless.

And then there was Andy Roddick, hilarious as usual.

And here’s India’s very own Leander Paes, taking the challenge to next level:

Federer has also tagged Indian men’s cricket captain Virat Kohli in the video. Here’s hoping Kohli responds.