NEW YORK — With a sure-to-be-hyped rematch with Serena Williams in the offing at the U.S. Open, Sloane Stephens held up her end of the bargain. Then Stephens — and the tennis world — needed to wait several hours to find out whether Williams would, too. From the moment the women's draw came out at Flushing Meadows, it was clear which potential fourth-round match was the most intriguing: defending champion Williams against up-and-coming talent Stephens.