Britain's swimming medal hopes were dealt a major blow when Ellen Gandy went out in the heats of the 200 metres butterfly.
Gandy had come to the London Olympics as one of the medal favourites.
The 20-year-old won silver at the World Championships in Shanghai last year and her form as well as her mature, quiet confidence, had given rise to optimism she may become the first British woman to claim an Olympic butterfly podium place.
The Melbourne-based swimmer faded dramatically on the final length though and finished fifth in her heat, 17th overall and one place outside qualification.
Two days ago Gandy finished eighth in the 100m final.
It was not all bad news for Britain as Gandy's good friend Jemma Lowe made it through to the semi-finals in the same event, in third place overall, while Michael Jamieson set a British record as he and Andrew Willis impressed in the 200m breaststroke heats.
The men's 4x200m freestyle relay squad reached Tuesday night's final.
Gandy's exit though was a real shock, with an Olympic medal the only one missing from her collection.
Qualification looked a formality until the final turn from which she emerged well down and clearly struggling, the tank obviously empty, down the last 50m as she touched in two minutes and 9.92 seconds.
That time was almost four seconds down on her swim in the same pool in March at the Olympic trials.
Her effort was reminiscent of the 2009 World Championships when Gandy entered as the world rankings leader but finished 15th over 200m and 16th in the shorter race.
A teary-eyed Gandy was at a loss to explain her performance today, saying: "I felt okay at the start but after 100m felt like I had been hit by a truck.
"I've been feeling fine, no health issues, I just don't know what happened."
Lowe, seventh in Shanghai last year, is a good friend of Gandy's, the pair having been close since the days of the Smart Track programme pioneered by former national performance director Bill Sweetenham.
Of Gandy, Lowe said: "She made the Olympic final in the 100m and that was brilliant, and she had a fantastic year last year.
"Everyone has ups and downs and I'm sure she gave it her best."
The Swansea ITC swimmer swam a measured race to touch in 2mins 7.15secs and is well capable of reaching the final.
Lowe said: "I'm really happy to make it into the next round, it's been pretty hard for me the last few days watching all my team-mates compete.
"I'm used to competing on the first day in the 100m. That's the best time I've done in the morning."
In complete contrast to Gandy's exit was the new record set by Jamieson as he became the first British man to dip below 2mins 9secs in the 200m breaststroke.
The Scot has risen to the occasion this week, setting a huge personal best over 100m when he just missed the final having only been drafted into that event last month following the withdrawal of Daniel Sliwinski.
However, his speciality is the 200m and his 2:08.98 today saw him break Kris Gilchrist's British record as well as take 0.86 off his previous best to clock the second fastest time overall.
The 23-year-old said: "I'm pretty happy with it, a little surprised with the time to be honest which is always good.
"Before the first swim in the meet you are always a bit apprehensive trying to see what kind of form you are in but I had a really good swim in the 100 and that's given me a lot of confidence.
"Being in the first seeded heat it's always difficult to pitch where you are and what you need to make it through so I had to set a marker and I'm absolutely delighted with it and fingers crossed I've got a little bit more to give tonight.
The Glasgow-born swimmer is one of a number of Scots, also including David Carry, Caitlin McClatchey, Robbie Renwick and Hannah Miley, who have made their mark in London.
He said: "It's always great, I think we've had a pretty good meet so far. Hopefully we can have another good one tonight."
Not to be outdone was his Bath ITC team-mate Andrew Willis who won his heat in 2:09.33 for the third fastest effort this morning edging out the champion in both 2004 and 2008, Kosuke Kitajima.
He said: "I am really happy with that for a morning swim. Looking at the times before mine it was tough out there and I knew I would have to go 2:10 so I wasn't going to mess around.
"I think the same time tonight will get me comfortably into the final."
The pair, who are both coached by Dave McNulty, clearly have the mental strength to rise to the occasion here and Willis insists there is no pressure with Rebecca Adlington's bronze the sole British medal so far.
He said: "I don't think so, the key is to get in a final and as you have seen this morning the crowd is really starting to light them up so the atmosphere is amazing.
"I can hear it down all the lengths, and especially the last, hopefully both of us can get in the final and it's anyone's game then."
Renwick, sixth in last night's 200m freestyle final, anchored the 4x200m freestyle relay to fifth place overall and a spot in tonight's final in 7:10.70.
Team member Ross Davenport said: "It was incredible, this crowd is absolutely fantastic and it's such an honour to race in front of such brilliant support."
Men's freestyle sprinting in Britain is in a real trough at the moment.
Only Adam Brown competed in the 100m freestyle heats today from which he failed to make it through to the semi-finals, finishing 20th in 49.20.