Koubek, a former top-20 player now ranked 118th, fought back after losing the first set to down world number 11 Ancic 3-6, 6-4, 6-3 in a two-hour thriller at the Cricket Club of India courts.

Koubek's semi-final opponent on Saturday will be third-seed Tomas Berdych of the Czech Republic, who brushed aside sixth-seeded Bjorn Phau of Germany 6-3, 7-5.

The other semi-final will feature top-seeded Tommy Robredo of Spain against fourth-seed Dmitry Tursunov of Russia.

World number seven Robredo overcame a late fight by Ramon Delgado of Paraguay before winning 6-2, 7-6 (7/1).

While Tursunov, world number 22, scored a hard-fought 3-6, 6-3, 6-4 win over British qualifier James Auckland.

The 29-year-old Koubek, who led 5-2 in the decider against Ancic, wasted two match points in the eighth game before wrapping up the match by holding his serve in the ninth.

"I think I was the fitter player towards the end of the match and that helped me win," said the Austrian. "Mario is a good player but except for a few players anyone can be beaten on a good day.

"I am really confident about going far in this tournament because I am playing quite well."

The defeat may have ended Ancic's bid to finish the year among the top eight and qualify for the season-ending Masters Cup in Shanghai in November.

"I have a few more events before that, so let's see how they go," he said. "Shanghai is obviously the aim."

Berdych, who was serving for the match at 5-4 against Phau, dropped the game, but broke back immediately and held serve in the 12th to win the match.