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By winning, Pierce finished first in the Black group and will play top-ranked Lindsay Davenport in Saturday's semi-finals.

Mauresmo was a set and a break up before Pierce began her fightback.

Mauresmo netted a backhand volley to get broken at 3-1 in the third, before Pierce won on her third match point.

GREEN GROUP 1 Sharapova: 2 wins, 1 loss 2 Davenport: 2 wins, 1 loss 3 Schnyder: 1 wins, 2 losses 4 Petrova: 1 win, 2 losses BLACK GROUP 1 Pierce: 3 wins, 0 losses 2 Mauresmo: 2 wins, 1 loss 3 Clijsters: 1 win, 2 losses 4 Dementieva: 0 wins, 3 losses Top two qualify for semi-finals

Mauresmo had already joined Pierce in the semi-finals by beating Elena Dementieva 6-2 6-3 on Thursday.

An exhausted defending champion Maria Sharapova, who meets Maursemo in the last four, lost her final group game 6-1 6-2 to Nadia Petrova.

Sharapova beat Lindsay Davenport in a three-setter on Thursday to guarantee top spot in her group and admitted that match had taken a lot out of her.

"This is the kind of tournament where all the soldiers are down and it's just whoever can get up," she said.

"I can't sleep. Throughout the whole day I'm like a walking zombie."

Kim Clijsters won her final group match when she beat Elena Dementieva 6-2 6-3.

The contest meant little with neither player having done enough to progress to the semi-finals, although at least it gave Clijsters her first victory.

Clijsters started the tournament with a chance to end the year as world number one after a Tour-best 67 wins.

"I still find it very hard to believe everything I've achieved this year," she said. "Every win means a lot more as I didn't have anything last year."