Rezai was serving for the match at 5-3 in the second set before reigning Australian Open champion Williams rallied to win five straight games to level the match and go a break up in the third.
Rezai recovered breaks in the first and seventh games of the third set but Williams broke her to lead 5-4 and then served it out.
Story continues below ↓advertisement | your ad hereWilliams, winner of 11 Grand Slam singles titles, will play Friday against the winner of the other semifinal between Olympic champion Elena Dementieva and sixth-seeded Victoria Azarenka.
The Australian Open starts Monday in Melbourne.
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