Agnieszka Radwanska.

Agnieszka Radwanska. Photo: Getty Images

The longest best-of-three-set match in WTA Championships history has won Agnieszka Radwanska a semi-final place in the $4.9 million year-end tournament for the first time. The bad news for the world No.4? A rested, eager, ominous Serena Williams awaits.

After Sam Stosur's 6-0, 6-3 flogging from Maria Sharapova in the dead rubber that raised the curtain on the two matches that would decide who joined Williams and Sharapova in the last four, Radwanska defeated French Open finalist Sara Errani 6-7 (6-8), 7-5, 6-4 in a three-hour, 29-minute marathon that surpassed by five minutes the record set in the 2007 final between Sharapova and Justine Henin in Madrid.

Neither is a power player; this was about subtley and variety, defence and dexterity, with 91 points decided at the net. At stake was what for Radwanska will be a Wimbledon finals rematch, but while Williams has not lost a set in her three matches, Radwanska has twice played for more than three hours. Petrol gauge? The red light may be imminent.

"Well, I will do everything in my power to do as much treatment as possible and to go on court tomorrow in one piece,'' said the understated Pole. "Pretty much I have nothing to lose playing Serena. She's a great champion and playing great tennis last couple weeks or months. So I'm just going to go on court and enjoy.

"She's playing also very good here, and especially I think she's more fresh than I am right now. So, well, will be tough for sure, but the surface is for sure different than last time we played. For who's going to better, we'll see tomorrow.''

Errani had been widely tipped as an 0-3 candidate from the white group, but the seventh seed used her topspin-heavy game, court smarts and strong volleying skills to good effect on her TEB-BNP Paribas WTA Championships debut.

"I'm here playing in the Masters with the best players of the year,'' she said. "It's good results for me. I won one match; this is one was very tough. I just try everything on the court, try to give my best, give everything, and this is it.''