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."
The 23-year-old has swum four of the five top times in 2012 but anyone who thinks gold is a formality should take heed of Lotte Friis, just nine hundredths of a second behind.
Friis was second behind Adlington when the Briton triumphed in a final-length shootout at last year's World Championships as she wrested the title from the Dane.
Friis, who was locked out of the medals as Adlington took bronze in the 400m earlier this week, has said she hopes to "spoil the party" and she is a real threat to Adlington's ambitions.
Friis said: "It was really easy and in control, I am satisfied with that race and hopefully I will be even better than that tomorrow.
"I hope the times will be even better and better than the World Championships.
"I hope I can improve on my bronze in Beijing. Rebecca is the main competition, she's the home crowd favourite and Olympic champion.
"I am not underestimating her in any way."
Ledecky announced herself on the world stage in June when she won the United States trials in 8:19.78, the third fastest time in the world this year.
This morning the teenager touched in 8:23.84 to qualify third fastest.
She said: "It was my first Olympic race, so I went in with a lot of adrenalin and a few nerves, but it carried me through the first 400m and I finished with a solid time."
There was a shock when world bronze medallist Kate Ziegler failed to make it through.
The American revealed tearfully she had been suffering with flu all week which saw her crash out in 21st.
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."
Proud and Simmonds will swim again tonight after making it through in the 200m backstroke.
Proud is based in Florida along with fellow backstroker Gemma Spofforth as well as American Elizabeth Beisel, who was second overall.
She was realistic after she went through in 11th in 2:10.01, saying: "I'm just pleased to get through, that was the main thing. I had to treat it like it was my final.
"Tonight I have to treat it like it's my final again.
"There is no reason why I can't get into the final although it's going to take a lot faster time."
Simmonds was sixth in Beijing but has not fared so well recently.
However she is safely through in 14th in 2:10.37, and said: "It's probably not the best race I've ever done but it was all about getting a race under my belt and making it back to the semi tonight and I did that by the skin of my teeth.
"I am pleased to have made it back and hopefully can improve on that tonight."
In the 100m butterfly, Rock was 24th in 52.56 with James 31st with a time of 53.25 while Brown was 20th in the 50m freestyle in 22.39.