A cartoon mocking Serena Williams for losing the US Open has sparked controversy and been accused of "racism". The drawing, published in an Australian newspaper, shows an angry Williams jumping on a broken tennis racket, as her rival in the final Naomi Osaka is asked by the referee: "Can you just let her win?" Williams received three warnings for coaching, smashing her racket and calling the umpire a thief in the tense match on Saturday. She was docked a game and fined £13,000 ($17,000), eventually losing to Japan's Osaka - by 2-6, 4-6. But the cartoon has sparked anger online. My toon in today’s @theheraldsun on #SerenaWilliams @usopenpic.twitter.com/didwtQg1R5 — Mark Knight (@Knightcartoons) September 10, 2018   Author JK Rowling responded to cartoonist Mark Knight. She wrote: "Well done on…