NEW YORK (REUTERS) - American Alison Riske recovered to beat twice Grand Slam champion Garbine Muguruza 2-6 6-1 6-3 at the US Open on Tuesday (Aug 27) and set up a second-round clash with another major winner Jelena Ostapenko.

Riske was her own worst enemy at the outset, committing 15 unforced errors and four double faults in a shaky first set, but settled down and finished with 39 winners to the Spaniard's 24.

Former world number one Muguruza, who also fell in the first round of Wimbledon this year, sealed her fate with a forehand into the net on match point for her 37th unforced error.

"I'm really proud of the way I fought through the entire match even though the first set didn't go as planned," the 29-year-old told reporters.

"I played a couple of sloppy games on my serve and I knew going into the second that if I was going to have any chance of coming back I was going to have to manage them a little bit better," she said.

"I just stayed present and tried to bring the fight. It worked today."

Muguruza, who parted ways with her long-time coach Sam Sumyk after Wimbledon, said there were no more easy matches with the raft of new talent and the staying power of savvy veterans.

"I remember not even that long ago, a few years ago, it was different. You had a difference between the top players and not top players," she said.

"Now you feel like if you're not 100% every day you have matches - like today, you know, all your opponents can play great. It's much more equal."

Riske will have her work cut out when she faces 2017 French Open champion Ostapenko for the first time on Thursday after the Latvian won 6-3 7-6(7) against Serb Aleksandra Krunic.