Serena Williams has won three of the last four Grand Slam titles but is still number two in the world, behind Dinara Safina, who has won none of the big four events of tennis.

Such are the intricacies of the WTA Tour rankings system that Safina can be fairly confident about keeping her seat at the top of the ladder at least until the US Open.

Not that Williams is complaining. "I'm happy with my results and winning Wimbledon," the American said.

"I guess I needed to win Rome and Madrid and I could have done better at the French. But I can't complain."

Safina's successes have come outside the Grand-Slam events and last weekend the Russian won her third title of the year, in Portoroz, Slovenia.

Her lack of head-to-head success against Williams, and sister Venus, however, makes her unworthy of the world number one ranking, says Jelena Jankovic.

"I think I'm the best player and I should always think like that but if I had to pick someone after me, I'd pick Serena," said Jankovic, who was the 2008 year-end number one and is now sixth. "Serena moves well, is strong and more complete. To be number one, you should be complete and if you are number one you have to be beating the Williams sisters," Jankovic told Reuters.

"I'm one of the rare players who has a positive record against the Williams sisters. Safina has beaten...them twice. If you want to be number one, you have to be up there with them.” Safina, who is not playing at Stanford, has lost six of seven encounters with Serena. Against older sister Venus, the world number three, she has lost three times in four meetings.

Serbian Jankovic has the upper hand against Venus Williams, with five wins and four losses, and has beaten Serena three times in seven matches.

WTA Tour officials said Safina's ranking was safe at least until mid-August and it would take an extraordinary run by Williams and a miserable stretch by Safina for the American to overtake her by the US Open, which starts on Aug 31.

Rankings are computed weekly on a cumulative points total over the previous 52 weeks, with higher-tier tournaments offering more points.

Going into the Stanford Classic, Serena was 1,742 points behind Safina. To jump to the top she would have to win here and go most of the way through, if not win, the premier events at Cincinnati and the Canadian Open next month. Safina would also have to lose early.

Part of the reason for the disparity between the two women is that, despite her Grand-Slam success, Serena, who had a run of knee and thigh injuries, pulled out of premier tournaments at Indian Wells and Charleston in March and April, receiving zero points in her ranking total.

Between Miami at the end of March, when she reached the final, and the French Open which started in late May and where she lost to Svetlana Kuznetsova in the quarter-finals, Williams played only three matches at four tournaments, earning seven ranking points.

For her part, Safina reached the final in Stuttgart and won back-to-back titles in Rome and Madrid, earning 2,120 ranking points. Plus, she reached the finals of the Australian Open and the French Open, as well as the semi-finals of Wimbledon.