In her first match since winning the U.S. Open in September, Williams defeated Angelique Kerber 6-4, 6-1 Tuesday to start round-robin play in the season-ending event featuring the top eight players in the world.
"Obviously, I would rather win majors. I've been No. 1 and I would love to be No. 1 again. I think maybe one day I will. We'll see,'' Williams said. "I just feel if I keep winning tournaments, majors, tournaments like this, and playing well and consistently, the ranking will come.''
In White Group matches, Agnieszka Radwanska defeated titleholder Petra Kvitova 6-3, 6-2, and Maria Sharapova defeated Sara Errani 6-3, 6-2 in a repeat of the French Open final.
Sharapova's win means she can still wrestle the No. 1 ranking from Victoria Azarenka, who plays Wednesday.
Williams won Wimbledon this year before adding the Olympic gold medal and the U.S. Open title. Between injuries and a curtailed schedule that generally includes a break after the U.S. Open, the third-ranked American had little chance of moving up in the year-end rankings.
The winner of 15 Grand Slam titles has only twice finished the year as No. 1 - in 2002 and 2009. She has played in the season finale six times and won twice, in 2001 and 2009, her final appearance at the event.
Kerber handed Williams one of her four losses on the tour this year- out of 54 wins - in the quarters at Cincinnati.
In a match of two heavy hitters, Williams allowed Kerber to tie it 4-4 after taking a 3-0 lead.
"I am just feeling my way around, pretty much,'' Williams said.
Williams yelled in delight after firing a backhand winner down the line for a 5-4 lead. She wasted two set points before Kerber put a forehand into the net.
Williams broke for a 3-1 lead in the second and, pumping her fist and yelling "Come on,'' she went up 4-1.
She lost the first three points of the final game, but played a superb volley to begin the rally and clinched the match when Kerber fired a forehand wide.
"I really wanted to win today, obviously. When you are playing anyone in the top five, you just are trying to play better,'' Williams said. "She won the last time we played, so I really just wanted to do well today.''
The fifth-ranked Kerber ended last year at No. 32, but a place in the semifinals at Wimbledon and two titles have made her the first German to qualify for the year-end championship since 2001.
Kvitova entered the match with a 25-match winning streak indoors but she was in trouble early, losing the first three games. Although she recovered to draw 3-3, Radwanska broke serve again and served out the set.
Kvitova dropped her first two service games of the second, and broke back once. But the former Wimbledon champion was unable to threaten the steadier Radwanska.
"I am just happy that I could play my best tennis today and close that match in two sets,'' Radwanska said. "It's a good start.''
It marked only Kvitova's second loss - and her first in four matches against Radwanska - in 28 indoor matches over two years. She was undefeated in winning the WTA Championships last year.
"I was nervous all match,'' Kvitova said. "I had a lot of errors.''
Kvitova is still in contention because of the round-robin format, with the eight players competing in two groups of four.