Williams, who claims to be exhausted near the end of a long season, reacted with fretful worrying, and sometimes distracting behaviour while struggling to a 6-4 2-6 6-4 two-hour win over Jankovic.

It included weeping into her towel, gesticulating and arm-waving, bending over and slumping, yelling and calling out, and both moving during the rallies and conducting the game at different speeds.

"I never saw her do this when she's leading,'' Jankovic said of Williams' go-slow moments.

"You know, every time she has a lead, she's running for the balls and she's hitting those big serves.

"For some reason, every time she starts losing, she starts serving slower or not running for certain balls. That's something that as a player you have to pay attention.

"It's not the first time when we played that she's doing this. That's her way of playing - or maybe when she plays against me. I don't know.

"But when it's an important moment, a huge serve comes up, so you have to be on alert every time. You never know if she's going to come slow or really fast one.

"It's not easy to focus. It's important for me to focus on my game and not really let those kind of things affect me and not allow me to play my game, but it's the way it is.

"She's the number one player in the world, and she plays so well. She deserves to be in this spot. But I think also when you play, winning or losing, you have to be up there and be a good sportsman."

Nothing in the rules of tennis says a player shall not play at different speeds.

However Jankovic appeared to be suggesting there are ways of conducting yourself not covered in the rules but which are eloquent of your attitude to the game.

Williams plays her last match of the 2013 season against Li Na in the WTA Championships final on Sunday.