French Open champion Li Na was upset 6-2, 5-7, 7-6 by Czech qualifier Petra Cetkovska in the semi-finals of the New Haven Open on Friday.
Cetkovska, ranked a career-high 40th, will face world number one Caroline Wozniacki in Saturday's final, the Dane having beaten Italy's Francesca Schiavone 7-6, 6-3 in the last four.
Cetkovska clinched the decisive tiebreak 11-9 at the end of a battle with Li lasting two hours and 41 minutes.
The 26-year-old Czech has yet to win a WTA event but is on an upward curve, reaching the fourth round at Wimbledon this year after winning three ITF titles last season.
