Jessica Pegula called her display "shoddy" and "erratic" as her Australian Open run ended in the semifinals to Elena Rybakina on Thursday.
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The sixth seed, bidding to win a first Grand Slam crown aged 31, did not drop a set in reaching her maiden Melbourne semifinal.
But the American went down 6-3, 7-6 (9/7) to Kazakh fifth seed Rybakina, who will play world No 1 Aryna Sabalenka in Saturday's final.
Pegula said that apart from at the end when she was battling to stay alive, she did not play to her full potential.
She admitted she struggled in the slower evening conditions on Rod Laver Arena, having played most of her matches at the tournament in the daytime.
"Honestly, I'm disappointed in my level," said Pegula.
"I felt like I didn't play very well until maybe the very end, and even at the end it was still kind of shoddy.
"I missed a couple balls, I had a lot of chances often. I felt like both of us were a little back and forth at times.
"She was just playing a little bit cleaner than I was."
Pegula felt she was especially erratic at the start, as she struggled to get used to the slower conditions.
She was always playing catch-up after that, against a player with a big serve and someone she praised for keeping ice-cool under pressure.
"I think I was just a little erratic in the beginning, kind of maybe overthinking it a little bit too and not just trusting the things that I had been doing as well," said Pegula, the beaten 2024 US Open finalist.
"But, you know, the conditions definitely make it tougher, and I just didn't adapt as quick as I needed to."