Sharapova, who received a wild card to appear in her first Grand Slam event since serving a 15-month doping ban, struggled at times with the counterpunching, moonballing Kenin, who committed less than half of the Russian's 33 unforced errors.
Zverev, who reached the Australian open quarterfinals in January, has a chance to match that feat at Flushing Meadows if he can beat his next opponent and the last American man left, Sam Querrey.
The 19-time major tournament champion, who stays in Manhattan during the U.S. Open, told reporters he chose the park court because it didn't involve a long car ride.
Not too long ago, Stephens appeared to be a rising star in tennis, making at least the fourth round at six consecutive Grand Slam tournaments, including a semifinal run at the 2013 Australian Open.
It was only the 19th match of the year for Kvitova, who missed the first half of the season after she was cut on her left hand - the one she holds her racket with - during an attack by a knife-wielding intruder at her home in the Czech Republic in December.
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