New York, Aug. 29 -- World No.15 Andrey Rublev was made to sweat before quelling a spirited fightback from American qualifier Tristan Boyer 6-3, 6-3, 5-7, 7-6(4) in a gripping second-round encounter at the US Open here today.

Rublev, who appeared in cruise control after pocketing the first two sets with ease, was pushed to the brink by the 23-year-old Boyer, a former Stanford University player who had only four tour-level victories to his name prior to his New York campaign. Backed by a raucous Grandstand crowd, Boyer stunned Rublev by stealing the third set with aggressive shot-making, capped by a forehand winner down the line.

The American, smiling gleefully on the changeover, had the momentum as Rublev left for a bathroom break. In the...