The 2006 champion of this event, Kuznetsova was attempting to reach her third final in as many events, having finished as runner-up at Dubai and Indian Wells.
She relinquished her serve on a fourth break point in the eighth game when she netted a backhand to put Williams in a position to serve out for the match.
The Russian did her best to recoup the service break but at deuce, Williams hit an ace to set up match point and then Kuznetsova sailed a backhand long.