John Isner brought the curtain down on his tennis career Thursday in a five-set loss to fellow American Michael Mmoh in the second round at the US Open. The 38-year-old Isner surrendered a two-set lead as world number 89 Mmoh saved a match point and came back to win 3-6, 4-6, 7-6 (7/3), 6-4, 7-6…