The No. 1-ranked and No. 1-seeded Williams has faced American opponents in all three of her matches this week while extending her overall winning streak at Flushing Meadows to 17.
Defending champion Sam Stosur was pushed for the first time in this year’s US Open by American Varvara Lepchenko, but dominated the second set after winning the first in a tiebreaker.
The four-time Grand Slam champion lost to 18-year-old Laura Robson of Britain in the second round of the U.S. Open on Wednesday, and will head into retirement after she finishes playing in doubles at Flushing Meadows.