PARIS (AFP) - Since Belgium's Justine Henin spent the last of her 117 weeks as women's world number one before retiring in May 2008, the top ranking in women's tennis has changed hands no less than eight times. Where once legends like Chris Evert, Martina Navratilova and Steffi Graf would hold onto top spot for what seemed like years on end, the honour has acquired a transitory quality that makes it difficult to establish who really is the best in the world. Russia's Dinara Safina is the current number one, having ascended to the position at the end of April.Safina, the younger sister of former men's number one Marat Safin, has 11 WTA career titles to her name, but has appeared out of her depth on the only two occasions she has managed to reach Grand Slam finals. She was dispatched 6-3, 6-3 by Serbia's Ana Ivanovic in the 2008 French Open decider and was completely out-classed by Serena Williams in the final of this year's Australian Open, losing 6-0, 6-3 in a chastening 59 minutes. "We all know who the real number one is," said Williams, the current world number two, at the Italian Open earlier this month. "Quite frankly, I'm the best in the world," added the American winner of 10 Grand Slam titles.Safina, though, hit back at her Melbourne conqueror by asserting that she has not had the time to prove her credentials as number one."She can say this because she's won many more Grand Slams than me," the 23-year-old said."But she's also older than me, so she has more experience. Let's see when I'm her age how many titles I have and then we can say."Safina's challenge now is to avoid the fate of the last handful of women to have risen to the top.Ivanovic became world number one on the back of her success against Safina at Roland Garros last year, but her form deserted her in the second half of the year as she slumped to her current position of world number eight. Her compatriot Jelena Jankovic took up the baton, ending 2008 at the top of the pile, but the 24-year-old lost her grip on the top ranking when she crashed out of the Australian Open in the fourth round and has struggled for consistency ever since. With Maria Sharapova having only just returned to action after being sidelined for nine and a half months with a shoulder injury, the Williams sisters have been given free rein to boss the Grand Slams. Venus won last year's Wimbledon, with Serena prevailing at the US Open in September and the Australian Open in January, and the Florida-based siblings will go into this year's French Open ranked two and three in the world. Serena has spoken of her desire to repeat her clean-sweep 'Serena Slam' of 2002-03 - when she won all four majors in succession - but she has come under criticism for failing to honour WTA tour commitments. The 27-year-old recently pulled out of the Madrid Open in the first round, citing a desire to keep herself in shape for Roland Garros, and has slammed the WTA for, as she sees it, forcing players to play even when injured. Navratilova, who won 18 Grand Slam singles titles, believes the Williams sisters must do more to boost the profile of the tour. "You have to support the tour. For years the Grand Slams have been getting too much attention," the Czech great told AFP. The 52-year-old, though, feels the absence of a dominant force in women's tennis is a good thing for the game. "People are more ready to criticise (the women's game) now. For example, when Chris (Evert) and I were dominating, people said there was not enough competition," she said. "Now you have so many different Grand Slam winners, people are saying the sport does not have any superstars. What do you want?"