So with the WTA Finals now just a week and a half away, here's 10 need-to-know facts about the defending champion.
1) Nine and counting
Williams will be competing at the WTA Finals for a ninth time. She made her debut at the season-ending showpiece, then held at Munich's Olympiahalle, in 2001.
2) In search of number five
In her previous eight visits, Williams has won four titles (2001, 2009, 2012 and 2013). Only two players have won more titles at the Finals - Steffi Graf (five) and Martina Navratilova (eight).
3) No.1 ranking up for grabs
After disappointing showings at the year's first three majors, Williams reasserted her dominance of the WTA with a magnificent summer, capped off with a third straight US Open crown. Despite these performances, Maria Sharapova's win in Beijing means that she can still pinch the year-end No.1 ranking from her American rival.
4) Top seed
One thing that is out of Sharapova's hands, though, is the seeding - Serena will line up as top seed.
5) Hunting down Martina
Williams has made the tournament her own in recent years, triumphing in 2012 and 2013. In fact, having missed out through injury in 2010 and 2011, she is unbeaten at the event since a round robin loss to her sister, Venus, in 2008. This run stretches 15 matches and is only bettered by Navratilova, who won 21 in a row between 1983 and 1987.
Victory in Singapore will also see Williams become just the third player to win three consecutive WTA Finals. The others were Seles (1990-1992) and Navratilova (1983-1986).
7) Making history
If Williams leaves Singapore with the silverware, she will achieve something not even Navratilova managed: completing the US Open-WTA Finals double three years on the trot. Navratilova (1983-84), Monica Seles (1991-92) and Graf (1995-96) all came close to doing so, each winning both tournaments two years in a row.
8) Age no barrier
Since hitting her 30s, Williams has been playing some of the best tennis of her career. In Singapore, she will be 33 years and one month old and will be looking to break her own record as the oldest winner of the tournament.
9) Injury worry
While Williams seems to be getting better with age, she goes into the WTA Finals with a few concerns over her fitness. She retired from her opening match in Wuhan with a viral illness and then withrew from her Beijing quarterfinal with a knee injury.
Williams is bidding to win her seventh WTA title of the year. She has only ever won seven or more titles in a campaign three times: in 2002 (eight), 2012 (seven) and 2013 (11).