Russia's Maria Sharapova let slip three match points as she crashed out of the WTA Finals in Singapore and lost her chance to become the year-end world number one on Friday.
The Russian, needing a straight-sets win to stay alive, won the first set 7-5 but she passed up the match points in the second before Radwanska won a tense tie-breaker 7/4.
Although the third set was yet to unfold, it brought an end to a disappointing campaign for the world number two who lost her first two group matches against Caroline Wozniacki and Petra Kvitova.
The 2004 champion also cannot overtake Serena Williams and end the season as world number one, in a boost to the 18-time Grand Slam-winner whose campaign is also hanging by a thread.
Williams, shattered 6-0, 6-2 by Simona Halep in her second group match, needs the Romanian to win at least one set against Ana Ivanovic later to reach the semifinals.
Meanwhile Radwanska is also relying on the result of Wozniacki's final round robin match with Kvitova if she is to make the semifinals of the eight-player season finale.