By Rory Carroll NEW YORK, Sept 7 (Reuters) Karolina Pliskova brushed off losing her number one ranking, but said she failed to live up to her own high expectations at the U.S. Open where she exited in the quarter-finals on Wednesday. Pliskova was beaten 7-6(4) 6-3 by American CoCo Vandeweghe to end her hopes of clinching a maiden grand slam title. The big-serving Czech had previously said that being number one came with additional pressure to perform at the tournament, where she lost in the final last year to Angelique Kerber. "I don't care," she said when asked her feelings on relinquishing the rankings top spot she held for eight weeks to Spaniard Garbine Muguruza. "I don't think something is going to change if you are number two. I don't see any difference between this. So for me, no changes." Pliskova never quite found her groove at Flushing Meadows this year as her ferocious serve lacked its usual bite. The typically reserved 25-year-old even saw her temper boil over during her match on Wednesday when she threw her racquet violently to the court in disgust after hitting a forehand long to hand the first set to Vandeweghe. "I think overall now I can say I was not playing my best tennis this tournament," she said at a news conference. "Still made it to quarter-finals, which is pretty good. "Just happy that this tournament is over now," she added with a smile. She said she will take away lessons from the tournament including learning how to win when not playing her best and how to cope with pressure. "This pressure, what I had here, this tournament, it was something new for me. So I never felt like this before. I was never actually defending a final of a grand slam. So it was totally different," she said. "Maybe I'm going to feel better on the next grand slams." REUTERS AKC0406