Martina thinks Serena needs to tone it down

Martina Navratilova, an 18-time Grand Slam singles champion, thinks Serena Williams was wrong in her outburst at the US Open women's finals, but agrees that a double standard was in play.

In an op-ed piece for the New York Times on Monday, the 61-year-old Czech-born American said a higher standard needs to be observed after Williams called chair umpire Carlos Ramos a "thief" and was penalized a key game in the second set against Japan's Naomi Osaka.

"We cannot measure ourselves by what we think we should also be able to get away with," Navratilova wrote.