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.
Radwanska, Halep and Pennetta finished with one win and two defeats each in the Red Group, but Radwanska advanced because she won three sets in her matches and the others two.
World number four Sharapova, playing her first competition since Wimbledon following injury, had already qualified for the semi-finals.
"I'm a little bit surprised that I've been able to win three matches, as physical as some of those matches were," said the five-time Grand Slam champion.
The final matches in the White Group take place on Friday with Garbine Muguruza, Angelique Kerber and Petra Kvitova competing for the two remaining places in the last four.
World number one Serena Williams is absent from the eight-player tournament in Singapore as she recovers from injury.