Great Britain's eventing team suffered a major setback when William Fox-Pitt was forced out of the competition because of an injury to his horse.
Fox-Pitt had been in fifth place overall after a clear ride with Tamarillo during the cross country.
But the horse injured its stifle - the joint in the hind leg similar to a human knee - and the duo withdrew.
It means Britain dropped 10 points and slipped back to fourth place while France lead ahead of Germany.
"The injury is not serious but horse welfare is always a prime consideration," read a statement from the British Olympic Association.
And GB team leader Will Connell added: "Of course the whole team are extremely upset for William, Tamarillo and the owners of the horse.
"William had an outstanding result when riding at Badminton this year and this is a very unfortunate way to end his Olympic campaign.
"I am sure we will see William and Tamarillo returning to the world stage - but this is a setback."
Great Britain had started the day in first place ahead of France and Germany after the dressage section and had high hopes of success.
Like Fox-Pitt, Leslie Law and Mary King also put in a good rounds and King's score now becomes part of the scoring British team - as the top three
riders only count towards the team event.
But Pippa Funnell found the going tough on Primmore's Pride.
Funnell hit the fourth fence hard and she eventually finished in 41st place to slip down to eighth overall.
And she said the relatively compact Athens course had not suited her horse.
"It wasn't just the length, it was the terrain," she said.
"He is a big long-striding horse and it is hard for him to stay balanced. I couldn't just let him keep running, he is far too precious for that.
"I am all for a 13-minute cross country course with big imposing fences. If this is the route we're going down, we have to change our approach."
However, France had a very good day, despite a fall by Arnaud Boiteau, with European champion Nicolas Touzaint, Jean Teulere and Didier Courreges all incurring no time penalties to stay on 113.4 overall.
The Germans suffered a scare when the experienced Ingrid Klimke fell during her round, but she remounted her horse Sleep Late and finished in the quickest time of the day.
They are now on 119.60 points, just over six points behind France on 113.40.
Ireland moved up two places to seventh on 191.0, thanks to Mark Kyle's clear round.
Touzaint retains his lead at the top of the individual standings on 29.4 with Germany's Bettina Hoy on 35.6 and American Kimberly Severson on 36.2.
There was bad news for the Belgian team when one of their horses, Over and Over, broke a bone in its left hind leg during the cross country.
The horse, ridden by Joris Vansprongel, fell at fence 26 and was immediately taken to the veterinary clinic.
X-rays revealed Over and Over had fractured its left distal femur.