World number one Justine Henin struggled to a 3-6, 6-3, 7-5 win over Czech teenager Nicole Vaidisova on Saturday to reach the Zurich Open final.
The Belgian next faces 19-year-old Frenchwoman Tatiana Golovin, who ousted Italy's Francesca Schiavone 6-0, 6-4.
"I didn't play a good match but there are always days like that where you have to accept you can't always be at your best," Henin told reporters.
"She pushed me a lot and I'm sure she's going to be giving us a lot of trouble over the next few years. I improved slowly but surely and eventually came through."
Chasing her ninth title of the season, and fourth in a row, Henin's run of 18 consecutive victories was in danger of ending against Vaidisova.
The unseeded Czech, who lost all three of her previous meetings with Henin, was under plenty of pressure at the start but the 18-year-old world number 15 stunned Henin by breaking serve to take a 5-3 lead.
If the teenager was nervous she did not show it, serving out to love as Henin dropped her first set of the week.
HOLD SERVE
The Belgian rallied to take the second set after finally breaking serve in the eighth game.
The two players failed to hold serve in the third set until Henin held for a 4-2 advantage.
She then spurned two match points at 5-3 before going on to lose her own serve. Henin, though, broke again to complete victory after two hours 18 minutes.
The unseeded Golovin had no such difficulty in the day's second semi-final, whipping through the opening set in 23 minutes with three breaks of Schiavone's serve.
The Italian recovered in the second set to hold her first four service games but then threw away the fifth by slamming a simple-looking overhead into the net.
Serving to win the match, Golovin faltered for the first time, offering Schiavone three chances to break back.
The French world number 18 saved all three with two cross court winners and a smash.
