Sania Mirza after her match against Patty Schnyder on Friday. (AFP)

Stanford: Sania Mirza recorded her second big upset in successive days as she packed off fourth-seeded Patty Schnyder of Switzerland in straight sets to storm into the semi-finals of the $600,000 Bank of the West Classic.

Unseeded Sania downed Schnyder 7-6 (7-2), 6-1 and set up a semi-final clash with eighth seed Austrian Sybille Bammer. Bammer advanced with a 6-2, 6-1 win over wild card Lilia Osterloh of the United States.

Sania had been in top form in the tournament and accounted for sixth-seeded Frenchwoman Tatiana Golovin for a place in the quarter finals.

World No. 17 and last year’s finalist Schnyder got off the blocks in a flash and raced to a commanding 5-2 lead before Sania came back hitting some powerful strokes and pushed the set to a tiebreak which she pocketed 7-2.

Cheered on by a big crowd, Sania was into her elements in the second set and disposed Schnyder 6-1.

“Experience and maturity are two things that helped out tonight,” said Sania, who reached the semi-finals in Cincinnati last week.

“When I am on the court, the crowd does get me going. This crowd is always cheering and there is always a positive vibe. But at the end of the day, you are the player and you are down 2-5, so you have to get it done,” Sania said.

Sania, however, said beating two players ranked in top 20 does not mean that she is going to take her semi-final opponent Sybille, whom she has never played before, lightly.

“Just because I was able to beat Golovin and Schnyder does not mean the road gets any easier tomorrow. Sybille is a great player and she is in the semi-finals so she has been doing things right too.

“She (Sybille) has also been to a final this year. The only advantage maybe is that I played a left-hander today and now I will be playing another left-hander tomorrow,” said the world No. 35.

In the other semi-final, top-seeded Russian Anna Chakvetadze will take on third-seeded Slovakian Daniela Hantuchova.

Chakvetadze rallied to beat seventh-seeded Katarina Srebotnik of Slovenia 4-6, 6-2, 6-2 while Hantuchova advanced with a 7-5, 6-3 victory over qualifier Olga Govortsova of Belarus.