Maria Sharapova advanced to the last four of the WTA Finals with an unbeaten round-robin record after outclassing Flavia Pennetta 7-5 6-1 in Singapore.
The Russian's third straight win ended the career of the 33-year-old Italian, who announced after her US Open victory that this would be her last season.
Sharapova's win secured Agnieszka Radwanska's place in the semi-finals.
Earlier, Radwanska beat top seed Simona Halep 7-6 (7-5) 6-1 but needed Sharapova to win in straight sets.
The Pole was two points away from losing the first set - and being eliminated from the tournament - but fought back to clinch victory.