Sania Mirza continued her impressive run this season, scoring an upset victory over third seed Mara Santangelo of Italy to enter the last four at the WTA Tier IV event in Pattaya, Thailand, on Friday.

The fifth seeded Indian won the quarter-final 6-4, 7-6.

She will face Gisela Dulko of Argentina, a 6-2, 6-3 winner over Australia's Nicole Pratt, in the semi-finals on Saturday.

Austria's Sybille Bammer and Shuai Peng of China will contest the other semi-final.

Sania twice came from behind to reach her second semi-final of the year. After a shaky start and an on-off service game, the 20-year-old fifth seed rallied from 1-4 down to take the first set and pushed the Italian to a tie-break in the second.

The Indian ace said she spent much of the match trying to work out her opponent's complex game.

"She's very difficult to play against, I don't know any player like her and I was trying to figure how she plays and her strengths and weaknesses," Mirza said.

"I had some bad serves but I got them right when I needed them. I was twice down but I always play better when I'm losing. It seems to motivate me to play better."

Mirza will meet Gisela Dulko in Saturday's semis after the Argentine sixth seed eased past Australian Nicole Pratt 6-2 6-3.

Austrian Sybille Bammer took almost three hours to get past Slovakia's Martina Sucha 3-6, 6-3, 6-3 to reach the second WTA semi-final of her 11-year professional career.

Bammer broke Sucha's serve in the opening game but lost the set 6-3 after two long games during which the pair were stuck at deuce 10 times.

Bammer fought back to take the second after some closely fought rallies and pulled away in the third to register her first victory against the Slovak in six matches.

"I got a little down when we were stuck at deuce all the time because I didn't take my chances but I got through."

Bammer faces fourth seed Shuai Peng in the semi-finals after the Chinese beat Israel's Tzipora Obziler 6-3, 6-2.