Fourth-seeded Agnieszka Radwanska reached the WTA Championships semi-finals on Friday but needed three-and-a-half hours to beat Italian seventh seed Sara Errani in a record-setting clash.

Radwanska fought back to win 6-7 (6-8) 7-5 6-4 – the longest match in the history of the year-ending tournament, according to the WTA.

Radwanska will face Serena Williams in Saturday’s semi-finals.

Maria Sharapova swept past Sam Stosur 6-0 6-3 earlier on Friday to retain a small chance of finishing the year as the No.1-ranked player in the world.

If Victoria Azarenka beats Li Na in the last match of the round-robin stage, however, she will be assured of staying No.1.

Sharapova had already qualified for the semis of the elite, eight-woman event and has won all three of her round-robin matches.

The Russian rolled through the first set, allowing Stosur only five points. Stosur was an alternate who replaced defending champion Petra Kvitova, who withdrew because of illness.

“I certainly didn’t want to play as long a match as I did in the previous round. I wanted to do a better job of a few things and I think I did,” said the second-ranked Russian.

Stosur put up more of a fight in the second set, but Sharapova ended it with a smash.

Sharapova won the French Open this year and briefly held the No.1 ranking.

Errani finished the tournament with one win and two defeats. The seventh-seeded Italian won a game in the middle set that went for almost 15 minutes but was not able to use that momentum and dropped her serve to fall behind 6-5.

Radwanska served out the set and opened up a decisive 3-0 lead in the final set to secure a place in the semis.

Williams was undefeated in three matches in her Red Group and didn’t play on Friday.