SINGAPORE (AP) Agnieszka Radwanska beat fourth-seeded Petra Kvitova 6-2, 4-6, 6-3 Sunday at the WTA Finals to clinch the biggest title of her career.
The fifth-seeded Pole's previous best at the season-ending championship was reaching the semifinals in 2012 and 2014.
''Definitely I didn't expect it at all,'' a tearful Radwanska said.
Radwanska is the first player to win the tournament after a losing 1-2 record in the round-robin stage - and the first Polish winner.
''I lost first two matches, but it's not like I was playing bad,'' Radwanska said. ''Then I think I got used to conditions, used to the surface.''
Radwanska led by a set and 3-1 in the second before Kvitova settled into the match to win seven of the next eight games.
''I really played well in the second when I came back and tried to push her a little bit,'' Kvitova said.
The Czech led 2-0 in the decider before Radwanska fought back to 3-3 and then won the last three games to take the title in 2 hours, 5 minutes.
Prior to this week, Radwanska hadn't beaten a top five player all season. This week she defeated three in a row on the way to the title: No. 2 Simona Halep, No. 3 Garbine Muguruza and No. 5 Kvitova.
Radwanska is the first WTA Finals champion not to have previously won a Grand Slam title since Amelie Mauresmo in 2005. The Frenchwoman won the Australian Open and Wimbledon the following year.
Kvitova, a two-time Wimbledon champion, now holds a 6-3 career record against Radwanska. Five of their meetings took place at the WTA Finals with Radwanska leading those encounters 3-2.