Canada's Eugenie Bouchard admitted she was not at her best Thursday despite beating Sorana Cirstea in the U.S. Open.
Bouchard advanced to the third round at Flushing Meadows for the first time but was made to work by Cirstea. She won 6-2, 6-7, 6-4.
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Bouchard will now face Barbora Zahlavova Strycova, the tournament's 30th seed. She admitted she must improve to go deeper in the tournament.
"I didn't feel as sharp as I want to be," she said.
"I was happy I could still play pretty solid but I didn't feel like I was at my highest level and she started playing very solid in the second and getting a lot of balls back and at the end of the day it was a fight and a battle."
Bouchard reached the semifinals of all three grand slam events before the Open, capped by a finals appearance at Wimbledon. She is the tournament's seventh seed. Cirstea, unseeded, is No. 80 in world rankings.
Bouchard won her Open debut last year before losing in the second round.
If Bouchard makes it to the quarterfinals, she could face No. 3 Petra Kvitova in a rematch of the Wimbledon final.
"I definitely hope I can improve from this match because I am definitely not totally satisfied."
It was the first time Bouchard had played on the Arthur Ashe court. She insists she was not overawed.
"It was cool, it was my first time on Ashe and it was special," she said.
"It was really cool that they played 'Genie in a Bottle' during the warm-up and I was like it is going well so far and this is pretty cool.
"They played it again at the end of the match and I felt pretty cool."