NEW YORK, Aug 31 (Reuters) - World number four Karolina Pliskova fired off eight aces to defeat American wild card Caty McNally 6-3 6-4 in the first round of the U.S. Open on Tuesday as she bids for an elusive maiden major. Pliskova, the 2016 runner-up, built up a 5-1 lead in the first set before her 19-year-old opponent settled her nerves, and hit 10 winners in the set compared to two from McNally, before breaking the American"s serve twice early in the second. McNally, who made it to the third round at Flushing Meadows a year ago, bounced back to level the score at 3-3, with the crowd in Louis Armstrong Stadium cheering her on, but could not match Pliskova"s power as the Czech closed out the match with a pair of aces. "I started really well then lost my focus a little bit at the end of the first set," said Pliskova, who lost to world number one Ash Barty in the Wimbledon final earlier this year. "She played (at) a really good level," she added. "She has a good future." Pliskova said she was pleased to keep the match to two sets as several top contenders, including twice champion Naomi Osaka and world number one Barty, stand between her and a Grand Slam title. She faces either Zarina Diyas of Kazakhstan or American Amanda Anisimova in the second round.