ISTANBUL, Turkey (AFP) — Even though Serena Williams is within sight of capping a tremendous comeback year by winning the WTA Championships, she already has half an eye on capturing another Grand Slam.

The Olympic, Wimbledon and US Open champion has finished top of her group without ever achieving her best, but aims to do that in today's semi-finals. Even if she can't, she says, she reckons she can next year.

Hence Williams already seems to have lateral vision of an increasing opportunity to add another Grand Slam title to her total of 15, at Melbourne starting in little more than two months' time.

"I can't wait to get to Australia," she said of the country where she has won five of her Grand Slam titles. "I wish it started next month."

She won't take a vacation, she says, during the off-season which starts on Monday — another sign that better form and more big titles is the highest priority.

"I've actually expected to play better," Williams said of the three comprehensive straight sets victories which put her top of her round robin group.

"If I could just play the way I have in practice it would have been more comprehensive in the matches.

"So hopefully as the tournament goes on I can get better. If not, I hope to do better next time."

These words create a sense that Williams may have reached a conclusion about the mysterious Tweet she posted last month.

"I have come to my crossroads of destiny," it said. "Choose wisely." Her choice seems to be that, despite her 31 years, she still has enough potential to try for yet further major ambitions.

Williams has already confounded those who thought her best days must be over, following a year out of the game with a blood clot which threatened her life.

This week both her words and her deeds have hinted that she has not given up hope of equalling the 18 Grand Slam singles titles achieved by Martina Navratilova and Chris Evert, her American compatriots, and perhaps even of trying to get closer to the open era record of 22 won by Steffi Graf, the German.

Meanwhile, Williams is enjoying a well-timed rest day with some light practice, while her next opponent will have to battle it out to earn the privilege of getting to the semi-finals.

It is most likely to be Agnieszka Radwanska, the Wimbledon finalist from Poland, who was playing her last group match yesterday against Sara Errani, the French Open finalist from Italy.

Later the top-seeded Victoria Azarenka needed to beat Li Na, the former French Open champion from China, to join Williams in the last four. If the Belarussian does that, she will also make sure of the year-end number one ranking.