While people all over the world are cooped up because of the coronavirus crisis, while trying to stay healthy and help others do the same, a bunch of athletes have been posting workouts and drills on social media with suggestions for staying in shape.

Federer did that sort of thing, too, with what he called a helpful solo drill last week, but he has also taken it a step further after he encouraged fans to tweet him their own videos mimicking the volleying exercise he displayed, before then replying to some, even dispensing a little advice.

ATP Montpellier
Goffin ends title drought with Montpellier crown

The owner of a men's-record 20 Grand Slam singles titles previously posted clips of himself hitting against a wall in the snow, including around-the-back or through-the-leg trick shots.

This time, the 38-year-old Federer donned an all-white outfit - perhaps a nod to Wimbledon, the grass-court tournament he has won eight times which was cancelled for 2020 last week - replete with a white panama hat with black band, standing near the green wall and volleyed against it. He tapped the ball more than 200 times during the test of reflex and form in the 59-second video.

Within six hours, Federer's clip garnered more than one million views, and his post drew more than 1,300 replies. True to his word, he answered some.

"Don't lean back, strong in the wrist," Federer wrote to one person. "Keep up the great work."

To another, in which a man hit a tennis ball against an indoor wall while a dog appeared to nap underneath, Federer answered: "Love the confidence not to drop the (tennis ball emoji) on the (dog emoji)".

To others, he sent verbal pats on the back, such as "good job" or "nice work" or "love the effort".

Federer, who had arthroscopic surgery on his right knee in February, is waiting along with everyone else for competitive tennis to return. The men's and women's professional tours are suspended until at least mid-July because of the COVID-19 pandemic.

Muguruza's coach tests positive for COVID-19 ahead of Qatar Open
ATP Rotterdam
Norrie wins in ATP Rotterdam first qualifying round