Rediff.com presents the early surprises at the year's final Grand Slam.
French Open champ Muguruza beaten by unheralded Latvian
Garbine Muguruza was once again left to sift through the remains of another failed US Open campaign.
The third-seeded Spaniard suffered a shock 7-5, 6-4 second round upset by unheralded Latvian Anastasija Sevastova on Wednesday.
The French Open champion has never found the same success on the Flushing Meadows hard courts as she has on Parisian clay, where she claimed her first Grand Slam title in May by beating Serena Williams in the final.
Zhang topples former champion Stosur in second round
China's Zhang Shuai, who broke through with a stunning run to this year's Australian Open quarter-finals, advanced to the third round on Thursday, beating former champion Samantha Stosur.
Zhang lost serve just once in ousting the 16th-seeded Australian 6-3, 6-3 on the Grandstand court.
The 51st-ranked Zhang became the first women's qualifier since 1990 to reach the quarters in Melbourne, in a surprise charge that came after she had failed to get past the first round of 14 successive majors dating back eight years.
Against 2011 US Open winner Stosur, who reached the semi-finals at this year's French Open, Zhang won a baseline battle by out-manoeuvring the Australian, forcing her into 27 errors, including 20 from the forehand side.
Raonic suffers shock defeat
American qualifier Ryan Harrison pulled off the biggest shock yet of the US Open by beating fifth seed Milos Raonic 6-7(4), 7-5, 7-5, 6-1 to reach the third round.
Raonic, who last month became the first Canadian man to reach the final at Wimbledon, arrived at Flushing Meadows looking to take the next big step in his career but his bid was cut short by the 120th-ranked Harrison.
The 24-year-old American, who had beaten Raonic once in their previous two meetings, came prepared to give the sixth-ranked Canadian a battle and proved the more durable during in a three hour 37 minute match, played under punishing conditions.
Wozniacki upsets Kuznetsova
Former world number one Caroline Wozniacki, one point from falling behind 5-0 in the opening set, rallied to upset ninth-seeded Russian Svetlana Kuznetsova in straight sets.
The unseeded Dane, slowed by injuries this season, took advantage of some sloppy play by the former US Open champion to post a 6-4, 6-4 victory and advance to the third round.
Ivanovic knocked out
Former world No. 1 Ana Ivanovic suffered a shock 6-7(4), 1-6 loss to Denisa Allertova of Czech Republic in the women's singles first round on Tuesday.
Ivanovic, the 2008 French Open winner, started off well but failed to maintain her momentum.
With the scores tied 6-6, the first set went into the tie-break in which the Serb star failed hold her nerve.
She completely collapsed in the second set, barely putting up a fight, going down 1-6.
Bouchard slumps to first round defeat
Eugenie Bouchard slumped to a first-round defeat on Tuesday, but the Canadian might yet leave Flushing Meadows a big winner with her lawsuit against the Grand Slam and United States Tennis Association still to hit the courts.
This year Bouchard had no one to blame but herself for her first-round stumble as she was tripped up 6-3, 3-6, 6-2 by 20-year-old Czech Katerina Siniakova, who registered a first ever win at the US Open.
Kyle Edmund upsets Richard Gasquet
Britain's world number 84 Kyle Edmund upset 13th seed Richard Gasquet on day one.
The 21-year-old Edmund, who helped Britain win the Davis Cup last year, crunched 10 aces and 40 winners in all en route to the biggest victory of his career.
Gasquet, who has reached the last 16 at Flushing Meadows four times in his career, was outplayed throughout and his US Open experience lasted just an hour and 41 minutes.