The third-seeded Williams dispatched Francesca Schiavone 7-6 (7-5), 6-4 in the night's first match and Clijsters prevailed over Australian Samantha Stosur 6-4, 5-7, 6-3 to set up a Friday semifinal showdown with Williams.
Schiavone forced a first-set tie-breaker but Williams claimed the last two points to win the set. In the second set, the Italian broke to pull to within 4-3 only to have Williams break back and move to match point. 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.
Williams is the last woman to win back-to-back U.S. Open championships (2000 and 2001) and is the only American remaining in either the men's or women's draws.
The second-seeded Clijsters won her 19th straight match in New York.
Stosur and Clijsters alternated four straight service breaks to get to 3-3 in the decisive third set, but the Belgian star then won the next three games to notch the victory and improve her record to 4-1 in U.S. Open quarterfinals.
Clijsters won the Open in 2005 and 2009. She and Williams have split 12 all-time matchups, but the Belgian has won four straight, including one this year in Miami.
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