American tennis player John Isner on Sunday fired the ATP Tour’s fastest serve of all time, clocking at roughly 253 kmph. Playing against Australia's Bernard Tomic in the Davis Cup World Group first-round tie, Isner fired 49 aces in the match which ended in Isner's favor with a scoreline of 6-4, 6-4, 5-7, 7-6 (7-4).