Kei Nishikori has suffered a surprise defeat to Benoit Paire in the opening round of the US Open.

The fourth seed, who reached the final at Flushing Meadows last year, squandered two match points on the way to losing 4-6 6-3 6-4 6-7 4-6 to the world number 41.

Ahead of this tournament, Nishikori had been struggling with an injury that forced him out of the Cincinnati Masters, and his lack of preparation resulted in a slow start against Paire, who took one of his four break-point chances to seal the first set.

However, Nishikori barely gave Paire a look in during the following two sets, conceding just 12 points behind his own serve to quickly establish control of the contest heading into the fourth.

Paire showed no signs of folding as he dug deep to take Nishikori to a tie-break, and despite facing two match points, he rallied to take the match into a decider.

With Paire only growing in confidence, Nishikori was in trouble and a solitary break of serve was enough for the 26-year-old to seal a remarkable victory.