Patrick Mouratoglou, Serena Williams' coach, took to Twitter on Thursday to call for an end to the in-match coaching ban.

"I have never understood why tennis is just about the only sport in which coaching during matches is not allowed," Mouratoglou wrote. 

He noted that coaching during play is allowed in every major team sport, as well as in individual sports such as boxing and golf. 

On-court coaching became a hot-button topic in tennis following Williams' controversial loss to Naomi Osaka at the 2018 U.S. Open final. Judge Carlos Ramos warned Williams when Mouratoglou coached her from the stands. The judge later penalized her a point for smashing her racket after she lost a game, and Williams would eventually lose a game for "verbal abuse" after she called Ramos a "thief." She also accused him of sexism after the match. 

The WTA lifted the ban on on-court coaching in 2008 during all non-Grand Slams. However, the four Slams continue to ban the practice, though it's often overlooked. WTA CEO Steve Simon has called for coaching to be allowed in all events. 

Mouratoglou wrote:

"It’s a very basic truth that the majority of tennis coaches are actually coaching on court, despite the rules. Look at how many times players look towards their boxes during a match. Some do it after every single point. Of course, coaches are usually discreet in the way they give messages to their players—they do it with signs or with loaded signals—but most of them also communicate verbally.

Occasionally the players are punished for it, but for the most part they are not, because the federations are perfectly aware of the situation and don’t ask umpires for a strict application of the rule.

For all those reasons, it is time the tennis leaders amend the status of coaching. It is time to authorize and structure—something the WTA has initiated in 2008 when it allowed on-court coaching for all tournaments apart from the Grand Slams—practices that have been tolerated all over the world for decades. This, I believe, would ensure that tennis sustains its development."

Mouratoglou admitted he was coaching Williams from his box during her match against Osaka. On the broadcast, viewers could clearly see him motioning for her to move up to the net.