As expected, the top seeds of both men's and women's competition in the 2015 US Open progressed to the second round, but Japan's Kei Nishikori suffered a first round exit after losing to Frenchman Benoit Paire.

The score read 6-4, 3-6, 4-6, 7-6 (6) and 6-4, which establishes Nishikori as the biggest casualty in the first round of the men's singles.

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Nishikori, who finished runners-up in the last edition of the US Open, looked below par against the Frenchman, who took full advantage after winning the first set. Nishikori did come back to win the next two sets and inch closer to a victory, but failed to convert his match point when he was leading 6-4 in the fourth set.

Paire clawed back into the fourth set and took it via tie break. Nishikori must rue the fact of missing out on an opportunity of winning the fourth set when he was so close. 

"I don't know," said Nishikori . "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."

When it boiled down to the deciding fifth set, Paire clinched it 6-4, thus establishing his first victory over the Japanese star. The Frenchman felt it was difficult to imagine getting past Nishikori.

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

Meanwhile, Djokovic was in ruthless form as he demolished his Brazilian opponent Joao Souza in straight sets with the score reading, 6-1, 6-1 and 6-1. The world number one is a major contender for the US Open title and with such a thumping performance, he has already sent warning signs to his fellow competitors.

Rafael Nadal, who has been impressive this season, defeated his Croatian opponent Borna Coric 6-3, 6-2, 4-6 and 6-4. The Spaniard was put to the test by the Croatian youngster, who managed to take one set from the former world number one.

In the women's arena, Serena Williams, who has won the last three editions of the US Open, was not even required to spend much time on the court as her first round opponent, Vitalia Anatolyevna Diatchenko, retired in the second set. The score then read, 6-2 and 2-0.

For big players such as Djokovic and Serena, the real competition starts from the second week, where they are expected to face some stiff competition.