Tennis legend Serena Williams has uploaded a video where she tells viewers she's now on day four of the Roger Federer challenge - hitting a ball against a wall non-stop.

And she also jokes that the only reason she's doing so well is because she's using a racket the Swiss master gave her and she 'can't let him down'.

Federer began the Stay at Home tennis challenge last week. It involves rallying a ball against a wall for as long as you can and thousands of people have sent him clips including players Novak Djokovic and Jalena Meyer.

Williams uploaded her clip a lot later than everyone else, leading to her claim that she had been doing the challenge ever since Federer set it.

She captioned the video: "4 days later... Couldn’t have done this challenge without my racquet from @rogerfederer. #tennisathome"

Williams, who is self-isolating for six weeks, has posted a series of social updates on how she's keeping herself - and it would seem others, amused.