5 biggest upsets at the US Open 2015

World No. 1 Serena Williams crashed out in the semi-finals of the OpenSeeds seemed to be just a number at this year's US Open as several top-ranked players suffered shock defeats at the hands of lower, or even unseeded competitors - the latter happening to World No. 1 Serena Williams, who lost to Italy's Roberta Vinci in the semi-finals of the US Open, ending her opportunity for a Calendar Grand Slam.She wasn't the only one, however. Here are 5 of the biggest upsets at the tournament, which concluded last week with Novak Djokovic and Flavia Pennetta taking the men's and women's titles respectively.

#1 Benoit Paire def. Kei Nishikori

5^th seed Kei Nishikori

Kei Nishikori , a finalist last year, was the first big seed to fall on the opening day of the tournament. The fourth seed who has had a fine year by already winning 3 ATP tournaments this year, could not recapture his fine form which he had displayed in 2014 against world no 1 Novak Djokovic by reaching the final.

Paire took the win 6-4, 3-6, 4-6, 7-6, 6-4.