Serena Williams is the first player to qualify for the season-ending Women's Tennis Association Championships and has already won seven titles this year, but the 31-year-old is still hungry for success.
As she prepared to make her first appearance of the North American hard-court season in Toronto, Williams said she would be satisfied only if she won every big tournament that she enters.
Although the world No 1 claimed her second French Open title two months ago, she suffered surprisingly early defeats in the two other Grand Slam tournaments, losing to Sloane Stephens at the Australian Open and Sabine Lisicki at Wimbledon.
However, she quickly returned to winning ways last month, landing a clay-court title in Sweden without dropping a set, and now has her sights set on retaining her US Open title.
"I'm not there yet with the way my year has gone," Williams said as she looked back on the first seven months of 2013. "Everybody is like: 'You've had a good year.' And I'm like: 'Who?' I work really hard to do the best, especially in Grand Slams."
Of Williams' 53 career titles, 15 have been won at Grand Slam level. It used to be the case that she regularly underperformed at lesser tournaments, but in the last three years she has been more consistent. Indeed, since the start of 2011 only three of the 16 titles she has won have been at Grand Slam events. Williams said she was enjoying her tennis more than ever this year.
"What makes tennis beautiful is the small tournaments that make you as a player and make you win the Grand Slams," she said.
"I was talking to [my sister] Venus the other day and I said, 'I am never going to stop.' I just love this sport so much – and we were just practising. It's just so fun. I'm having more fun than I have ever had."
While the leading women are playing in Toronto this week, most of the top men are competing at the Montreal Masters.
Novak Djokovic, the top seed, was due to play his opening match overnight against Germany's Florian Mayer, while Andy Murray will make his bow today against the winner of yesterday's late match between Grigor Dimitrov and Marcel Granollers.
Mayer earned his chance against Djokovic by inflicting more misery on Australia's Bernard Tomic, who has endured a difficult year, especially since his father and coach John was repeatedly banned from tournaments after an alleged fight with his son's hitting partner.
Tomic said after his 5-7, 6-3, 6-3 defeat: "I missed my chances, I didn't focus early on in the second set and that's when he took over. I dropped my concentration and he played very well."