The 33-year-old Pennetta, who was eliminated with the loss, was playing in her last career tournament after announcing at the U.S. Open she would retire at the end of the season.
''Right now I feel like it's not my last match,'' Pennetta said. ''I feel normal completely. I don't know why. Maybe in few days I will feel more the difference.''
Sharapova completed the round-robin stage 3-0, while Radwanska, Pennetta and Halep all finished at 1-2. Radwanska advanced because of more sets won, with Pennetta having to win at least one set against Sharapova to reach the semifinals.
Pennetta led 3-1 in the first set but Sharapova broke back in the eighth game and took control.
''I think to have the last match against Maria was amazing to play such a good champion,'' Pennetta said. ''Was a good way also to say goodbye, because when you lose against such a good player there is not too many things to say.''
Sharapova came into the tournament after being sidelined with a leg injury since the Wimbledon semifinals. At the Wuhan Open in China this month, she retired in her first match back with a left wrist injury.
''I wanted to try to play a high-quality three matches,'' said Sharapova, who won the WTA Finals in her debut appearance in 2004. ''I'm actually a little bit surprised that I've been able to win three matches as physical as some of those matches were.''
Halep reached the final in her first WTA appearance last year but became the first player to be eliminated.
''I go home,'' Halep said. ''I go to the holiday. It's OK. I think I had a good year this year.''
Radwanska trailed 3-1 in the first set and then 5-1 in the tiebreaker before turning the match around. In the second set, Halep saved a break point to hold serve in the first game but won only one point in three more service games.
''I was done,'' Halep said of the first set. ''No energy anymore. I was tired. I felt that I lost the chance to win the first set and probably I lost the chance to win the match in that moment.''