Comfortably into her third decade on tour, Serena Williams remains the dominant power in women's tennis with her eyes firmly fixed on overhauling Martina Navratilova and Chris Evert's Grand Slam record in 2013. The American, who will turn 32 in September, finished the year at three in the world. She was in a class of her own in the second half of 2012, winning Wimbledon, the Olympics, the US Open and then capped it off with a stunning straight sets demolition of Maria Sharapova in the WTA Championships final. "I definitely think I can improve some more," said Williams, who now has 15 Grand Slam singles titles. That's just three off the 18 collected by Navratilova and Evert with only Steffi Graf's record of 22 looking potentially out of reach. "The day I feel that I can't improve, I think that's the day I should probably hang up my racket," added Williams, whose success in the latter half of the year looked a distant dream in the first six months of 2012. Having missed 12 months of action in 2010 and 2011, battling life-threatening blood clots on her lungs, she was a fourth-round loser at the Australian Open. Then, despite winning claycourt titles in Charleston and Madrid, her stunning first round defeat to Virginie Razzano at the French Open was her worst career loss at a major and seemed to beckon the end of the line. But by the time Wimbledon came around in June she was rejuvenated, taking a fifth title at the All England Club in a final where opponent Agnieszka Radwanska at least went down fighting. It was also at Wimbledon where Williams, never far from controversy, celebrated her Olympic gold with an impromptu "crip walk", a dance linked with America's violent gang culture. After her trials in the first half of 2012, the American is reluctant to predict another "Serena Slam" where she would hold all four Grand Slam titles at the same time. To do that she would have to win both the Australian and the French Opens -- trophies currently held by world number one Victoria Azarenka and Sharapova, the world number two, respectively. Azarenka, 22, who pushed Williams all the way in the US Open final in September, finally came of age in 2012, claiming six titles and realising the talent that had often been at loggerheads with a dangerously frail temperament. Not that her feistiness has completely vanished. At Rome, neither she nor Sharapova would give ground at a changeover, a mutual stubborness which led to an unladylike shoulder barge at the net. With Williams, Sharapova remains the sport's biggest attraction. The 25-year-old Russian also dominates the rich list of women's sports -- according to Forbes, she has raked in at least $20 million dollars each year since 2007. Completing a career Grand Slam with her win at Roland Garros in June only served to boost the Russian's A-list profile even more. However, the year wasn't without setbacks when her romance with basketball player Sasha Vujacic ended. Sharapova is a more mature figure on and off the court, a legacy of her lengthy absence from the sport in 2008 and 2009 when a crippling shoulder injury saw her ranking plummet to 126. "I try to think that if you're level-headed about difficult defeats and then if you get something in your career that's incredible, you take it in a calm way, then the defeats don't seem as difficult," she said. While the likes of Sharapova, Williams and Azarenka will be back for more in 2013, former world number one Kim Clijsters will be deep into her second retirement. The 29-year-old triple US Open champion saw her career ended for good by British teenager Laura Robson at Flushing Meadows in August. "This feels like the perfect place to retire. I just wish it wasn't today," Clijsters said. "I gave it all and just wasn't good enough at the end of the match."