NEW YORK -- A remarkable US Open run that concluded with an appearance in the final last year changed the perception of Kei Nishikori, but a first-round loss Monday reminded why doubts have lingered about him.

A native of Japan, Nishikori, 25, entered this year's US Open with a career-high ranking of No. 4 and three titles, including Washington earlier this month. There was nothing to suggest he would struggle in Monday's opening round -- or that he ultimately would be done less than 3½ hours into the fortnight.

Frenchman Benoit Paire came back to stun Nishikori 6-4, 3-6, 4-6, 7-6 (6), 6-4 in Louis Armstrong Stadium for the certifiably biggest win of his career.

Before last year's tournament, Nishikori had a reputation for being a little soft around the edges. There were lingering injuries, awkward retirements and a string of unimpressive results in the Grand Slam events.

And then, in the narrow space of a fortnight here, Nishikori changed that nagging perception. He took down three top-10 players in a row, requiring 14 of the maximum 15 sets to do so. His fourth-round match against Milos Raonic ended at 2:26 a.m., tying the latest-finishing match at the Billie King National Tennis Center. His quarterfinal defeat of Stan Wawrinka also went four-plus-hours and led to an even greater upset in the semifinals, over No. 1 Novak Djokovic.

And although he lost in the finals to Marin Cilic, he left that US Open with a new place in the game. Soon he would rise to No. 5 in the world and appear for the first time in the ATP World Tour year-end championships.

However, on Monday, Nishikori shrank from a big moment in the fourth-set tiebreaker when he had a match point at 6-4. He somehow contrived to miss a forehand and went on to lose the last four points of the frame.

Nishikori said he lost his concentration after letting match point get away. How, exactly, does that happen to a professional player?

Kei Nishikori followed up last year's impressive run to the US Open final with a first-round dud. Streeter Lecka/Getty Images

"I don't know," he said in his postmatch news conference. "It was a long match, and it's never easy. I had forehand to finish, and I couldn't really make it. So it took a little bit long time [to regroup]. I shouldn't, you know, thinking too much.

"First match in Grand Slam, it's never easy to lose that opportunity. Took some time to get back in the match."

Paire beat Nishikori for the first time in three tries and beat a top-10 player for only the third time in 19 attempts -- and the first time in a Grand Slam.

"To beat Nishikori," he said afterward, "it was impossible for me to imagine."

The match's most telling blow? With Nishikori serving at 2-all in the fifth at 30-40, he steered an 87 mph second serve out wide, which Paire throttled. It was a 91 mph backhand, and it found the crosscourt corner. Paire issued a prolonged scream that underscored the shift in momentum.

He finished things with a monstrous 133 mph ace down the middle, his fastest of the match. Paire had an all-or-nothing bottom line: 64 winners and 67 unforced errors. Nishikori actually won more points, 160-156.

"It was more because of him," Nishikori said, but added, "but I wasn't playing 100 percent."

Unlike Nishikori, Cilic easily advanced Monday, earning a straight-sets win over qualifier qualifier Guido Pella.

Also advancing in the first round were top-ranked Novak Djokovic, who needed just 71 minutes to beat 91st-ranked Joao Souza of Brazil 6-1, 6-1, 6-1, and two-time US Open champion Rafael Nadal, who beat 18-year-old Borna Coric 6-3, 6-2, 4-6, 6-4 in Monday's last match.

"Very happy to be back," said Nadal, who missed last year's tournament with a right wrist injury, "and to be through."

Nadal said he felt dehydrated on the muggy night, and that's why his level of play dropped for about a set.

"I had some problems. I was sweating a lot," Nadal said.

Nadal said he "had a little bit of (a) stomach problem" and wasn't "physically perfect," which is why he lost the third set, before feeling stronger in the fourth. He said he didn't think it was a big deal and shouldn't affect him Wednesday in the second round.

American Mardy Fish also was an opening-round winner in what he has announced will be his final professional tournament.

"I'm glad I got to come back here one more time," said American Mardy Fish, a Monday winner in what he has said will be the final event of his career. Al Bello/Getty Images

Fish put away 102nd-ranked Marco Cecchinato of Italy in four sets. Fish is returning to the US Open for the first time since 2012, when he was forced to withdraw before a fourth-round match against Roger Federer because of a panic attack.

"I'm glad I got to come back here one more time," said Fish, who has since been diagnosed with anxiety disorder.

It was not as enjoyable a day for Frenchman Gael Monfils

A back injury forced Monfils, the 16th seed, to stop playing his first-round match against Illya Marchenko while trailing 2-6, 6-4, 5-0, 30-0.

Monfils's back has hindered him the entire summer, and he fell on it early in the second set Monday.

"I couldn't play, couldn't move, couldn't do anything," said Monfils, twice a quarterfinalist at the US Open and a crowd favorite at Flushing Meadows.

Information from The Associated Press was used in this report.