By Matthew Cronin

STANFORD, California (Reuters) - Belarussian Victoria Azarenka celebrated her 21st birthday in style by overwhelming top seed Samantha Stosur 6-2 6-3 in the Stanford Classic semi-finals Saturday.

Azarenka will face either three-times grand slam champion Maria Sharapova of Russia or third seed Agnieszka Radwanska of Poland in Sunday's final.

The eighth seed, registering her fourth victory over the Australian in as many attempts, tore apart Stosur's second serves and battered her from inside the baseline.

Azarenka broke world number five Stosur five times in the match and was a broken only once herself.

"I was very good on the return, putting pressure on her," said Azarenka, who reached her third final of the year. "I was very solid from the baseline and I was able to keep holding."

Former world number six Azarenka, who has battled with leg injuries for much of this year and saw her ranking dip to 18, said she was now healthy and she did look fully fit.

Stosur's big serve consistently let her down and she was only able to land 55 percent of her first serves. She was also constantly pushed around the court in rallies and committed 33 unforced errors to only 15 from her opponent.

"She played quite well and punished me when I didn't put a good serve in," Stosur said. "It wasn't my best day and I still had quite a few chances to get back in it, but when I had those I didn't come up with the goods."

Stosur is the first top five player Azarenka has beaten since she overcame Williams, who is now world number one.

"Sam is great player and has had amazing results this year," Azarenka said. "I'm very happy that I could beat her when she's in good form. It gives me confidence."

(Editing by Ken Ferris)