World number one Serena Williams’ comeback trail endured further feisty moments with Jelena Jankovic, the former world number one from Serbia, on Thursday before she reached the semi-finals of the Dubai Open.

They have exchanged testy words on several occasions, the last time in October when Jankovic appeared to question Serena’s sportsmanship at the WTA Championships in Istanbul.

This time, near the end of Serena’s impressive 6-2 6-2 win, a dispute blew up over the speed at which the rallies were conducted, a familiar issue.

It happened when Jankovic appeared to slow the pauses between rallies to the speed she thought Serena had been taking, prompting Serena to hold her arms out wide.

This caused Jankovic to walk towards Kadir Nouni, the French umpire, complaining at length, once gesturing towards Serena as she did so.

The match lasted another four points, when there was a further exchange of words between Jankovic and Serena.

Afterwards Jankovic suggested the umpire might have been at fault.

“First he tells me I’m serving too fast,” she said. “Then when I took my time. It was like one minute.

“So you never know what to do. Usually if I go over 25 seconds with whoever I play, you know, warning, time violation. Here it’s like you can do pretty much whatever.

“I have to follow who knows which rules.”

Serena was placatory. “I told her at the end I didn’t mean anything, and I’m sorry if I played slow.

“She was like, no – it was more that they say she plays too fast and then they say she plays too slow.

“She said she couldn’t get right.”

Serena will have an unseeded semi-final opponent in either Alize Cornet of France or Carla Suarez Navarro of Spain.

There could even be an all-Williams final for the first time in more than four years.

Elder sister Venus Williams continued to raise hopes of a return to the top echelon with her third straight sets win in a row to reach a semi-final against Caroline Wozniacki, the former world number one from Denmark.

The seven times former Grand Slam winner overcame Flavia Pennetta, the seed-destroying Italian, 6-4 7-5, with a mixed performance.

Asked how she had managed to recover from her illness and fitness problems, Venus, still ranked outside the top 40, said: “I pray and try a million different things all the time.

“Serena has been extremely supportive and my family have changed their life style for me. It makes me feel better.”