Great Britiain's Ellie Simmonds is expecting a "really tough race" as she looks to defend her S6 100m freestyle title.
Simmonds qualified as the second fastest, with Victoria Arlen swimming a world record time.
"I think it's going to be a really tough race tonight, [Victoria] Arlen has just equalled the world record [of 1:14:74] so it'll be really hard," Simmonds told BBC Radio 5 Live.
"I had an awful night sleep but just going here today to try and enjoy myself. I do get extremely nervous and sometimes think 'why am I doing this.'
"I wasn't nervous today but there are lots of emotions I don't want the competition to end and I just want to enjoy this."
Simmonds has already won two golds at London 2012.
