Top seed Samantha Stosur was involved in an intriguing tussle with Belgium's Yanina Wickmayer before finally escaping with a 7-5 3-6 6-3 quarter-final win at the Bank of the West Classic in California.

The first set stayed on serve up to 5-5 before Stosur finally created the match's first break point and took advantage go 1-0 up.

The seventh seed battled back, though, and broke her opponent twice to take the second, before Stosur came out on top from a topsy-turvy third.

Fifth seed Maria Sharapova reached the semi-finals with a three-set win over fellow Russian and second see Elena Dementieva.

Sharapova won the opening set but her opponent hit back in the second before Sharapova took command in the seventh game of the final set to triumph 6-4 2-6 6-3.

Next up for the Russian is a last-four clash with third seed Agnieszka Radwanska, who beat Russian Maria Kirilenko 7-5 6-0.

She will next face Victoria Azarenka, who beat French fourth seed and defending champion Marion Bartoli 3-6 6-3 6-3.

Bartoli had won seven consecutive matches at the tournament following last year's win, but could not make it eight tonight.