Rebecca Adlington produced the fifth fastest 800 metres freestyle in the world this year to send out an ominous warning to her rivals as Britain's women had a successful morning at the Aquatics Centre.
Ellie Faulkner failed to make it through alongside Adlington but Lizzie Simmonds and Steph Proud both made the 200m backstroke semi-finals.
However, no British men negotiated the heats with Antony James and Michael Rock out of the 100m butterfly and Adam Brown falling short in the 50m freestyle.
Adlington is seeking to become the first Briton to successfully defend an Olympic swimming title and her time of eight minutes 21.78 seconds has been bettered this year only by herself and American 15-year-old Katie Ledecky.
Adlington said: "I'm pleased with that. I didn't know what time I was going during that one at all.
"I was like 'this could be anywhere from an 18 to a 38'.
"I just put my head down and went for it especially after the 400 heat only scraping in. I was like 'I am not risking that even if I give it a little bit more than I should do, I'd rather get in safely', so I just went for it and I am pleased with that time.
"Obviously it's going to be a battle tomorrow but I've just got to see what I can get."
City of Sheffield swimmer Eleanor Faulkner was 22nd in 8:38.00 and said: "It was a hard tough swim, a great experience, I did as best as I possibly could but my time wasn't there today."