Stanford, California (Reuters) - Serena Williams has won three of the last four Grand Slam titles, but she is still world number two, behind Dinara Safina, who has not won any of the four major tennis events. Such is the complexity of the WTA Tour ranking system that Safina can reasonably be assured of keeping her top spot at least until the US Open. Not that Williams is complaining. "I am happy with my results and winning Wimbledon," the American said ahead of this week's Stanford Classic where she is the top seed. "I think 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, missing a head-to-head victory over Williams and her sister Venus, however, making her unworthy of being world number one, says Jelena Jankovic. "I believe I am the best player and I must always think so, but if I had to choose someone after me, I would choose Serena," said Jankovic, who was year-end number one in 2008 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 must beat the Williams sisters," Jankovic told Reuters. Diplomatic Venus is one of the rare players who has a positive record against both Williams sisters. Safina has beaten... twice. "If you want to be number one, you must be up there with them," Jankovic said. Safina, who is not playing in Stanford, has lost six of seven matches with Serena. Against older sister Venus, the world number three, she has lost three times in four meetings. Serbian Jankovic has the edge over Venus Williams, with five wins and four losses, and has beaten Serena three times in seven matches. Venus was diplomatic when asked whether her sister, who holds a total of 11 Grand Slam titles, or Safina deserved the higher ranking. "Both players have their results and know their results and we shouldn't put one player down and one up," she said. "Both have done their best." 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 streak by Safina for the American to overtake her before the US Open, which starts on August 31. Rankings are calculated weekly on a cumulative total of points over the previous 52 weeks, with higher-level tournaments offering more points. Going into the Stanford Classic, Serena was 1,742 points behind Safina. To pass her, she would need to win here and go most of the way through, if not win, the Premier events in Cincinnati and the Canadian Open next month. An injury opened up problems for the Beijing Olympic silver medalist Safina, who is defending points from 2008 titles won in Los Angeles and the Canadian Open, would also lose early. The reason for the disparity between the two women is that, despite her Grand Slam success, Serena, who had a series of knee and thigh injuries, withdrew from the Premier events at Indian Wells and Charleston in March and April, receiving zero points for her ranking total. Between Miami at the end of March, when she reached the final, and the French Open which began at the end of May where she lost to Svetlana Kuznetsova in the quarterfinals, Williams played only three matches in four tournaments, gaining seven points for the rankings. For her part, Safina reached the final in Stuttgart and won back-to-back titles in Rome and Madrid, gaining 2,120 points for the rankings. Moreover, she reached the final of the Australian Open and French Open, as well as the semifinals of Wimbledon. Elena Dementieva spoke up for her Russian compatriot. "We have been working very hard on our ranking system over the last few years and it is better," the world number four said. "The ranking system has improved and I think a lot of points come from winning a Grand Slam, so it is really strange to see a number one without winning a Grand Slam. It's very peculiar, but give credit to Dinara, she was winning so many tournaments (the others). She deserved to get this number one position, but I am sure that if Serena keeps playing like this she will get that position very soon." (Compiled by Clare Fallon)