CNN  — 

Benoit Paire has gained a cult following on the tennis tour for his fiery temper, fondness for fast food and guzzling soft drinks during changeovers.

But on the opening day of the U.S. Open, the Frenchman displayed his talent – and grit – in upsetting last year’s finalist Kei Nishikori in five sets after saving two match points.

Nishikori entered this season’s tournament somewhat in uncertainty, since he skipped the Cincinnati Masters, a warmup, with a hip injury. Yet not many would have forecast Monday’s 6-4 3-6 4-6 7-6 (6) 6-4 result in New York, since the diminutive but hard-hitting Japanese baseliner went into last year’s U.S. Open on the back of a toe injury and subsequently became the first Asian man to make a grand slam singles final.

Further, Nishikori began the match against the 41st-ranked Paire as the world No. 4, his highest ever ranking, and even when he failed to convert the successive match points in the fourth-set tiebreak, the odds remained on his side: He owned a stellar 12-3 record in fifth sets while Paire stood at 4-4, including three defeats alone at the U.S. Open.

Paire, though, registered the lone break of the fifth and on a stiflingly hot day in the Big Apple – similar to the conditions when Nishikori ousted world No. 1 Novak Djokovic last year in the U.S. Open semifinals as part of his breakthrough fortnight – sealed the three-hour, 14-minute contest with a 21st ace.

“Yeah, it’s very sad to lose always first round, but I think he was playing good tennis,” Nishikori told reporters. “So, I mean, I don’t think I played bad. Didn’t play great, but still, it’s never easy first match.”

Paire’s jubilant celebration was fitting for a man who so nearly tumbled outside the top 150 in February as the effects of a knee injury in 2014 continued to take a toll on his ranking.

“It’s a big victory,” Paire, whose diet is better nowadays, told reporters. “What can I say more? To beat Kei, he made final last year, so for me, yes, it’s my best victory during my career. But I don’t want to stop now. I don’t want to stop after Kei, after this match.”

On paper, he shouldn’t, as his next foe is 84th-ranked Marsel Ilhan of Turkey.

Ana Ivanovic, like Nishikori a grand slam finalist – she won the French Open in 2008 – also quickly departed.

Ivanovic faced an even tougher draw than Nishikori, landing 2014 Australian Open finalist Dominika Cibulkova, and lost 6-3 3-6 6-3.

The Slovak would have been seeded – and thus not encountered the world No. 7 in the first round – if not for a foot injury that sidelined her for three months. On Monday it was Ivanovic’s foot that was the issue, the Serb taking a medical timeout in the final set for blisters.

“It’s definitely very disappointing, today’s loss, because I felt like I did a lot of hard work over last few months,” Ivanovic told reporters. “Over the last week, you know, I had great preparation, lots of good training and everything.

“It was just unfortunate that we came up against one another so early in a tournament.”

Later, men’s No. 1 Novak Djokovic thrashed Joao Souza 6-1 6-1 6-1. Brazil’s Souza played – and lost – the longest Davis Cup singles match ever in March at six hours, 42 minutes but on Monday he lasted a mere 71 minutes against the nine-time grand slam champion.

If Djokovic, Ivanovic’s friend, reaches the final, he would become just the second man in the last 45 years, after Roger Federer, to make four grand slam finals in one season.

Women’s No. 1 Serena Williams, aiming to become the first player to win all four majors in a season since Steffi Graf in 1988, faces Russia’s Vitalia Diatchenko first up in the night session. Williams’ sister, Venus, needed three sets to get past Monica Puig, 6-4 6-7 (7) 6-3.

Djokovic features in the same quarter as the slumping 14-time grand slam winner Rafael Nadal, who meets 18-year-old Borna Coric in the second match of the night session following Williams’ clash. Coric, the youngest man in the top 50, defeated Nadal last October in Basel.

Marin Cilic may be under the radar, although he is the defending men’s champion. The Croatian downed Guido Pella in straight sets, 6-3 7-6 (3) 7-6 (3) and meets Russian qualifier Evgeny Donskoy in the round of 64.

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