On a Sunday afternoon in New York, a 23-year-old named Ben Shelton stepped into a moment American men's tennis had been waiting 23 years to reclaim — and came just short. Alexander Zverev, steadier and more seasoned, won the US Open final 6-3, 7-6, 5-7, 6-2, not by outplaying Shelton so much as by outlasting him. Yet in the long arc of a sport searching for its next great American voice, a loss can sometimes be the beginning of something larger than a trophy.