It was the Russian’s second victory of the season over Williams, having beaten her in the Wimbledon final as well a few months earlier.
In the tour championships, held in Los Angeles at the time, Sharapova took out Svetlana Kuznetsova (US Open champion that year), Vera Zvonareva, Anastasia Myskina (French Open champion in 2004) before dismantling Williams in the final.
She hasn’t beaten Williams since.
The world No2 hopes she can draw inspiration from her triumphant run at those championships, in order to put together a strong campaign in Singapore this week, where she could run away with the No1 ranking if she gets a good result and Williams gets a poor one.
“I couldn't believe that I was part of a field at that point in my career,” Sharapova recalls of her 2004 year-end championships.
“I was in Los Angeles where I had been training with Robert (Lansdorp) for so many years. It felt like a home tournament in a way for me. I remember the players. It was of course a very tough field as always. Just going through the draw there and the way that I felt and the way I played, I've seen some clips as well, very inspiring. Certainly hope I can do that here again.”
But not all of Sharapova’s finals at the championships had a happy ending. Three years later, the Russian suffered an agonizing defeat at the hands of Justine Henin, who beat her in three sets in the longest final in the history of the event.
“I remember being incredibly upset after that final,” says Sharapova. “It was one of the few times where I think it took me quite a long time to get to the press conference because I was really upset. I just wanted to win that match so much, because it was just one of the most physical matches I've ever played.”
She would then go on to make a third final in 2012, which she lost to Williams.
Sharapova, who begins her WTA Finals quest on Tuesday against Denmark’s Caroline Wozniacki, has never ended the year at No1, although she did spend three stints at the top of the rankings in 2005, 2008 and 2012.
Asked how much replacing Williams as No1 would mean to her, Sharapova said: “I've always experienced the joy of grand slam wins so much more, because the spur of the moment. There is actually a point that you have to win in order to get it, whereas the rankings will depend on other people's performances during the year, at certain tournaments.
“Is it an incredible accomplishment? Absolutely. It would be amazing to achieve that.”
Meanwhile, Wozniacki is set to end the year inside the top-10 for a sixth consecutive year - a record amongst all active players. The ex-world No1 is regaining her form thanks to a solid second half of the season, where she was runner-up to Williams at the US Open.
“I think my game is in great shape. I feel in great shape physically. I have fun playing out there on court,” said Wozniacki, who trails Sharapova 3-5 head-to-head but beat the Russian in their most recent meeting at the US Open. “I think all of that together kind of brings out the best of me when I'm out there competing.”
The other match of the day will see two-time Wimbledon champion Petra Kvitova take on world No6 Agnieszka Radwanska, who is making her sixth appearance at the Finals.
Kvitova, who won the title on debut in 2011, leads the Pole 5-1 head-to-head and 2-1 in WTA Finals clashes.