WHAT HAPPENED: Last year Kei Nishikori grabbed worldwide headlines with his surprising and historic run to the US Open final.

On Monday, Nishikori made more headlines but not the kind the Japanese star was hoping for, as the No. 4 seed tumbled out on the opening day to free-swinging, hard-serving Frenchman Benoit Paire in five sets.

Paire, No. 41 in the world, saved two match points in the tiebreak and made a service break in the fifth game of the final set stand up for a 6-4, 3-6, 4-6, 7-6, 6-4 triumph and his biggest career win.

“It was impossible for me to imagine this,“ the 26-year-old Paire told the Louis Armstrong Stadium fans at the conclusion of the three hour and 14 minute battle, played out in temperatures near the 90-degree mark. “I am going to go out and have a good time with my coach and friends now.”

There was little reason to believe that Paire could pull off such a huge upset. He had an 0-2 career mark against the rising 25-year-old sensation, who was coming off a strong summer campaign on the Emirates Airline US Open Series with a win at Washington and semifinals at Montreal.

The hard-serving 6-foot, 5-inch Paire saved his best for the final set when he dominated Nishikori on his service games, winning 73 percent of points on his first serve. Up 3-2 after a service break, Paire closed the door on the match emphatically, losing just two points in his last three service games. He finished the match with his 21st ace, a booming 133-mph first serve, the fastest of the match.

Paire was pretty much going for broke throughout the match, finishing with 64 winners, nearly double Nishikori's total.

Nishikori looked poised to escape with the win, leading 6-4 in the fourth-set tiebreak. But he lost the first match point after a lengthy baseline rally with a forehand wide, and Paire blew away the second with a 122-mph service winner. Paire grabbed a set point with another service winner and then the set on another Nishikori forehand miscue, winning the tiebreak 8-6.

WHAT IT MEANS: Paire’s victory might ease the way for his countrymen No. 19 Jo-Wilfried Tsonga and No. 16 Gael Monfils, who are both in what was Nishikori’s part of the draw. Nishikori’s loss also wipes out a possible quarterfinal repeat of last year’s US Open final against No. 9 Marin Cilic of Croatia. Up next for Paire is Marsel llhan of Turkey, who advanced when Radek Stepanek retired in the fourth set.

QUESTION: After last year’s breakout season, Nishikori was being tabbed as a possible winner of a major this season. Will this loss put a damper on talk of his prospects to win a major?

August 31, 2015 - Benoit Paire in action against Kei Nishikori (not pictured) in a Men's Singles - Round 1 match during the 2015 US Open at the USTA Billie Jean King National Tennis Center in Flushing, NY. (USTA/Ned Dishman)