Michael Phelps surprised even himself with a record-breaking start to his bid for an unprecedented eight Olympic gold medals as Britain's swimmers struggled on the opening day of competition in Beijing.
Phelps, who won six gold and two bronze medals in Athens four years ago, broke his own Olympic record in the heats of the 400 metre individual medley to cruise through to Sunday's final.
Teenager Hannah Miley, 19, was Britain's only swimmer to advance to an individual final in the women's event, while Chris Cook was 15th of the 16 qualifiers for the 100m breaststroke semi-finals and Jemma Lowe slowest of those to advance to the semis of the 100m butterfly.
Phelps was inside his own world-record pace after the opening butterfly leg and just 0.02 seconds outside it after the second backstroke leg.
And although the 23-year-old American dropped off record pace after the breaststroke, he still posted a record time of four minutes 7.82 seconds, 2.57secs outside his world record set earlier this year.
"I'm pretty satisfied with the time, I didn't think I'd swim that fast in the preliminaries," said Phelps. "All I know is that I wanted to be in the middle lane of the pool tomorrow."
Britain's Thomas Haffield, fourth fastest in the world this year coming into the Games, could only finish a disappointing seventh in his heat in a time four seconds slower than his personal best, while compatriot Euan Dale also missed out after finishing sixth in his heat.
Haffield was mystified by his poor performance, the 20-year-old from Neath saying: "I will have to go back and talk to my coach and go back to the drawing board.
"I think it was just one of those days. Everything was going great up until I got in. I got in the water and nothing happened. I did the first two strokes and it didn't feel right."
In the women's 400m individual medley, Miley - ranked fourth in the world this year - was the slowest qualifier for tomorrow's final as team-mate Keri-Anne Payne missed out.
Miley, who celebrated her 19th birthday on Friday, was second in her heat behind American world-record holder Katie Hoff and eighth quickest overall in 4min 36.56secs. Payne was fifth in her heat and 15th overall in a high-quality event.
"I knew it would be tough because everyone is swimming PBs just to make it into the final," Miley said. "But I'm over the moon to have made it.
"I just need to go away, get plenty of sleep, a good meal and come back tomorrow and give it my all."
In the women's 100m butterfly, Lowe and team-mate Francesca Halsall were in the last of seven heats but only Lowe made it through to Sunday's semi-finals as the slowest of the 16 qualifiers.
The 18-year-old from Hartlepool said: "It was the hardest race I have ever done, everyone was so close.
"I was trying to focus on myself and make it to the semis and I did it. I hope I can go quicker tomorrow. My personal best is 57.78 (today's quickest qualifier swam 57.58) so who knows what can happen. Tomorrow is another day."
Aberdeen's David Carry broke his own British record with a time of 3:47.17 in the 400m freestyle but still failed to advance to tomorrow's final.
The 26-year-old was fifth in his heat and 15th fastest overall with team-mate Dean Milwain seventh in his heat and 21st overall.
Carry said: "You come here dreaming of swimming fast in the Olympics and that's what I did today. I knew it was going to take a massive step up to make the final.
"I knew it was going to take 3:45 and I thought I was in the shape to do that but they pulled away from me at the end. I can't be unsatisfied with the way I swam. I was puking my way up the pool so I think that shows I was giving 100%!"
In the 100m breaststroke, Cook scraped into tomorrow's semi-finals by finishing fourth in his heat and 15th fastest overall.
Cook, one of just five men to break the one-minute barrier this year, clocked 1:00.70 but team-mate Kristopher Gilchrist failed to advance after managing just seventh in his heat in 1:01.34.
"I had a terrible start, I need to get in there and get going tomorrow," Cook said.
Norway's Alexander Oen set a new Olympic record of 59.41secs in winning the seventh heat - and the fastest five qualifiers were all inside the old mark - while Ireland's Andrew Bree was only 30th fastest overall despite setting a new personal best of 1:01.76.
In the final event of the session, Britain's quartet of Francesca Halsall, Caitlin McClatchey, Julia Beckett and Melanie Marshall made the final of the 4x100m freestyle relay as the slowest qualifiers in a time of 3:39.18.