Serena Williams's return to Flushing Meadows will be the headline, but one of this summer's top story lines has been the nascent rivalry between No. 1 Simona Halep and No. 3 SLOANE STEPHENS(left), last year's U.S. Open champion. Halep beat the American in the finals at Roland Garros in June and Montreal this month, but both matches were thrilling three-set slugfests. "She makes me play better and better every time we meet each other," Halep said after her most recent triumph. Is it a full-fledged rivalry? Maybe not yet. But whatever it is, it's heating up.