The US Open started right for some players, but it was also frustrating to some especially for Japanese Kei Nishikori and Serbian Ana Ivanovic.

Yahoo Sports said it was a total upset for Nishikori who is aiming to have a repeat appearance in the US Open finals just like last year.

Frenchman Benoit Paire defeated the No. 4 seed in an opening game on Monday with the score 6-4, 3-6, 4-6, 7-6, 6-4.

In an ABC report, Paire said it was hard for him to believe that he could beat the Japanese tennis player.

"To beat Nishikori, for me, it was impossible to imagine this. I'm really happy," said Paire, who scored 21 aces while Nishikori only fired three.

"I had a really good serve today. I saved two match points so I'm really lucky," the French player added.

It was also noted by ABC that the Japanese left Louis Armstrong Stadium "with his head down after his third first-round exit in New York in five years."

"I think he [Paire] was playing good tennis. I mean, I don't think I played bad. Didn't play great, but still it's never easy first match. I had matched point and kind of lost a little bit of my forehand." He added that he lost a bit of concentration, too," Nishikori said in the Yahoo Sports report.

On the other hand, Ivanovic was beaten in straight sets by Slovakia's Dominika Cibulkova.

"It's not easy, but today I felt like she used her opportunities better than I did," Ivanovic told Yahoo Sports.

According to Sports Illustrated, the No. 7 seed was aiming for a second Grand Slam title since her only major win was during the 2008 French Open.

In a Reuters report published by The Star, Cibulkova said she did well during the game.

"Today I know that my game was there on the court. I just had to stay tough and know I had to do it, she doesn't give away shots," the 26-year-old tennis player added.

This was also considered as redemption for Cibulkova who just had an Achilles-related surgery on her left foot earlier this year. She is set to face Jessica Pegula in her next game.

Meanwhile, Serena Williams was victorious in beating 25-year-old Russian Vitalia Diatchenko, whom she praised for the effort of fighting her during Day 1.

"I told her I was proud of her for coming out and making this effort, knowing she was injured. That's a great effort," Williams said in a BBC report.

Williams' older sister, Venus, also advanced to the second round of the competition after she defeated Monica Puig.