Swimmer Fran Halsall was the first British woman to make it into the pool in heat four of the 100 metre butterfly.
The Loughborough ITC swimmer's favourite event is the 100m freestyle which comes later in the week and she was second in her heat in 58.23 seconds and eighth overall going into the evening's semi-finals.
Ellen Gandy, fancied for a medal in the 200m butterfly after world silver last year, was also second in the following heat in 58.25, going through in ninth.
However, American Dana Vollmer made a statement of intent, the 2011 world champion setting a new Olympic record of 56.25, 0.19 outside Sarah Sjostrom's 2009 world record set during the shiny suit debacle.
Halsall said: "It's exciting actually because when my name came up on the board just before we were about to start, all I could hear was the crowd going, 'Whey!', and they're blowing the whistle for everyone to be quiet and everyone's still shouting, so it really made me smile and relax.
"The first one's always a bit more nervous than the rest because you want to know how you're going and how you're feeling and just blow the cobwebs away.
"So it's nice to have that extra bit of relaxation because I was giggling about the crowd shouting. So, yeah, I was very happy with that."
The 22-year-old admitted the butterfly was a warm-up for the free, saying: "Yeah, definitely. Just to sharpen up a bit and get a bit of race practise before my main one.
"So I just want to sharpen up, make sure everything's where it needs to be, soak up the atmosphere and just enjoy it - and I could do that with the fly. So it's perfect."
Halsall described the atmosphere as: "Amazing. Absolutely fantastic. I loved it."