Time marches by at a head-spinning rate. As she prepares for her 10th Wimbledon, starting tomorrow, Serena Williams has reason to pause and reflect. The 16 seeds in Williams' debut year at Wimbledon in 1998 included Steffi Graf, Monica Seles, Jana Novotna and Anna Kournikova, while only two of the seeds, Venus Williams and Patty Schnyder, are still playing today. Three years ago, when Serena missed the French Open and Wimbledon, there was speculation about her own future. Even when she won the 2007 Australian Open she was clearly short of her best physical shape, having played just five tournaments in the previous 16 months.

At 27, however, she has just enjoyed her most successful 12 months since 2003, when she completed the 'Serena Slam', holding all four Grand Slam titles at the same time. Reaching last year's Wimbledon final was the start of a run that saw her win the US and Australian Open titles and regain the world No 1 ranking she had last held five years previously. She has 10 Grand Slam titles and is hungry for more.

Williams needs hard work to stay in shape. She thinks she has never been fitter than this year, thanks in part to her winter training. "I was in Florida and a little bored – a big change from LA," she says. "I just trained with Venus and my Dad in Florida. I practised a little more than I normally do. Florida's a little boring. There's nothing to do there. Every day I felt like Bill Murray in 'Groundhog Day'. I couldn't wait to get to Australia. Maybe that's why I won – because I didn't want to go back.

"It was definitely a lot more focused for me in Florida. I'm on a great regime now – in LA as well. And being around Venus and seeing her work hard makes me want to do better and work harder. I have to work hard. I'm lazy. But, you know, the older I'm getting the more I'm enjoying hard work. I've realised that my job is to stay fit – and that's a pretty good job to have. I'm not a natural-born person who wants to go to the gym. That's just not me."

Does Venus bully her into working harder? "No, she just always says: 'Well, I did this, this and this.' It makes me want to say: 'Well, I did this, this, this and this.' So it was good. We were able to motivate each other."

For years the sisters appeared to hate playing each other. Their matches were often poor and there were suggestions – strenuously denied – that they took it in turns to win. Over the years, however, their contests have become gripping. Last year's Wimbledon final, which Venus won 7-5, 6-4, was memorable, while their US Open quarter-final, which Serena won 7-6, 7-6, was arguably the best women's match of the year, full of wonderful athleticism and ball-striking.

"I think our matches have got a little better," Serena says. "We've got better as well, so that has made the quality of the match better, and our games have got closer. At one point she was dominating me and then I was winning. Now we're really neck-and-neck [they have won 10 matches each]. I think that has something to do with it as well."

As for her own game, Serena believes she is a more complete player. How would the 2009 Serena fare against the 2003 version? "I think the Serena now would definitely beat the other Serena. I'm older. I think I'm wiser, a more mature player. Back then, it was more nitty-gritty. Now it's more composed and planned-out playing."

Her recapturing of the world No 1 position – since relinquished to Dinara Safina – was remarkable considering her comparative lack of success on Tour. In nearly five years she has won just four tournaments away from the Grand Slam events. Safina has won six in the last 13 months alone – but has lost three Grand Slam finals along the way. After the Russian became world No 1 earlier this year, Serena was dismissive. "We all know who the real No 1 is," she said. "Quite frankly, I'm the best in the world."

She insists, nevertheless, that losing the top spot is no big deal. "It's cool to be No 1, but everyone assumes I'm No 1 anyway. When I'm at home people come up to me and say: 'You're No 1.' I hadn't been No 1 for ever, but I never corrected them. I just let them assume. It's important, I guess, but at the same time I don't worry about it."

She concentrates on the Grand Slam tournaments. The Tour have tried to construct a calendar in which the top women all play at certain events, with fines for those who do not turn up, but appearing on court and winning matches are two different matters. Serena has fallen at the first hurdle in her last three tournaments outside the majors, losing to Klara Zakopalova, Schnyder and Francesca Schiavone.

Provided she remains fit, she plans to be around for a while yet. "I just feel like I'm going to play and keep playing. Right now I don't see an end. Today I actually enjoyed practising for the first time ever. I hate practising. I thought: 'Oh my God, the older I get the more forehands I want to hit.'"

And her ambitions for Wimbledon after finishing runner-up 12 months ago? "My goal is just to do better than last year."