New York: US teenager Frances Tiafoe came tantalisingly close to a stunning US Open upset on Monday, only to be denied by compatriot and five-set master John Isner.

Tiafoe, an 18-year-old who received a wild card into the draw of the final Grand Slam of the year, won the first two sets and served for the match in the fifth before the 31-year-old Isner sealed a 3-6, 4-6, 7-6 (7/5), 6-2, 7-6 (7/3) victory.