Garbine Muguruza felt she was well below her best during a shock third-round loss to Barbora Strycova at the Australian Open.
The Spanish third seed was stunned 6-3 6-2 under the Rod Laver Arena roof on Saturday, making a surprise early exit.
Muguruza had 32 unforced errors in her loss and said she simply put in a below-par performance against the Czech.
"Clearly not my best, for sure. I think today is a very bad day at the office. I think Barbora played good, very good. Also, I didn't play good," she said.
"But I think I just couldn't find the court, my shots. I didn't really find my game."
Asked about her condition, Muguruza said she lacked her usual energy, but paid tribute to Strycova - who had failed to take a set off her in two previous meetings.
"I felt with not so much energy, that's for sure. I felt a little bit, I don't know, not moving very well. I guess also Barbora was making me move. She was being smart," she said.
"Yeah, didn't feel quite good at any moment today."