Since the advent of computerised rankings in 1975, 25 women have climbed to the summit, but only 12 have finished the season on top.
Wimbledon champion Muguruza was deposed as world No 1 by Halep but she said she would have to be at her very best if she wants to win the year-end trophy and rankings crown on October 29.
"Honestly, I had no preference about who I played", the 24-year-old Spaniard said. A number of players can get that, and I wish it was me.
In 2008, Williams won the WTA Finals title, going 3-0 in the round-robin, and defeating Vera Zvonareva of Russian Federation in the final.
"I'm ready. I felt like I was going to be here (in Singapore), and here I am".
Muguruza feels fit, fresh and keen to bank on her experience of previous visits to Singapore to emerge from a group containing the tour's hardest hitters in the round-robin phase.
Maybe they were all being polite, but it appeared that no one wants to outrighly stake their claim for that spot.
American Sloane Stephens was named Comeback Player of the year.
Williams joins top-ranked Simona Halep of Romania, second-ranked Garbine Muguruza of Spain, third-ranked Karolina Pliskova of Czech Republic, fourth-ranked Elina Svitolina of Ukraine, sixth-ranked Caroline Wozniacki of Denmark, seventh-ranked Jelena Ostapenko of Latvia and ninth-ranked Caroline Garcia of France.
"So I'm just staying myself, staying humble at practice, keep improving, keep working out".
Germany's Angelique Kerber was presented the Jerry Diamond ACES Award, which goes to the player who consistently goes "above and beyond" in the promotion of women's tennis to fans, media and local communities.
"(Being No.) 1 is my dream come true, but you don't want to stress yourself too much", she said.
Halep faces debutantes Svitolina and Garcia in the Red Group round robin, along with the veteran Wozniacki, while Muguruza is in White Group with Williams, Pliskova and French Open champion Ostapenko. I'm happy that I got it in the past, and now I'm not that nervous or obsessed. "Is just the best feeling that I ever had in my tennis career so far", the 26-year-old told reporters in Singapore on Saturday.
The tournament proper starts tomorrow, at the Singapore Indoor Stadium.