Novak Djokovic emerged from an exhilarating and exhausting U.S. Open final with a 24th Grand Slam title on Sunday night, using every ounce of his energy and some serve-and-volley guile to get past Daniil Medvedev 6-3, 7-6 (5), 6-3 in a match that was more closely contested than the straight-set score indicated.