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Clijsters committed 31 unforced errors in the Black Group contest as Pierce hit back from 3-1 down in the final set to beat the Belgian 6-1 4-6 7-6.

In the Green Group, Maria Sharapova needed three sets to beat Patty Schnyder 6-1 3-6 6-3.

And world number one Lindsay Davenport, brushed past Russian Nadia Petrova 6-2 7-6 in the final match of the day.

Black Group rivals Elena Dementieva and Amelie Mauresmo get their campaigns under way on Wednesday.

The two top players in each group will advance to Saturday's semi-finals.

Pierce's victory gave her revenge over Clijsters after losing to the former world number one in the final of the US Open final in September.

"I was just hoping it would be better than the last match," said Pierce, who had never beaten the 22-year-old Belgian in three previous meetings.

"In the first set, Kim wasn't playing well. Something was bothering her and she wasn't moving. I was surprised how I won the first set quickly and easily."

Clijsters, who arrived in Los Angeles last Friday, said she was still overcoming jet lag.

"I never felt like I was quite there," she said. "The only thing I want to do is go to the hotel and go to sleep."

It is never easy not to play for a few weeks and come out and play the top eight players in the year Maria Sharapova

Sharapova was also a little off the pace.

The teenager has battled injuries in the latter stages of her 2005 campaign and has played just two tournaments since the US Open.

"It is never easy not to play for a few weeks and come out and play the top eight players in the year," said the Russian.

Although she got off to a quick start, taking the first set in 27 minutes, Sharapova made a string of unforced errors in the second set.

She appeared to regain control in the third, winning the first three games en route to a 5-2 lead.

But she squandered two match points against Schnyder's serve in the eighth game and had to fight off a break point in the next before getting a chance to serve out the match.

"I knew it was going be tough," said Sharapova, who briefly seized the world number one ranking this year.

Schnyder qualified for the WTA Tour Championships thanks to two tournament titles and three runner-up finishes. She also reached the quarter-finals or better at 15 of 25 events.