The third-seeded American star dispatched Schiavone 7-6 (7-5), 6-4, advancing to take on the winner of Tuesday's evening match between No. 2 seed and defending champion Kim Clijsters and fifth-seeded Australian Samantha Stosur.
Schiavone forced a first-set tie-breaker but Williams claimed the last two points to win the set.
In the second, the Italian broke to pull to within 4-3 only to have Williams break back and move to match point. But Schiavone prolonged the contest, pulling within 5-4 before Williams broke back again and won for the 52nd time in 53 career U.S. Open matches when taking the first set.
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