Keri-anne Payne was just edged out of a medal as she finished fourth in the 10-kilometre open water swim in the Serpentine.
Hungary's Eva Risztov was the winner, eight years after last competing at a Games in the pool, holding off a late challenge from American Haley Anderson.
Expectations had been high before the race that double world champion Payne could upgrade her silver medal from Beijing. The 24-year-old produced her customary start, going ahead from the off, but she was never allowed to settle and despite a desperate sprint to the line she finished 0.4 seconds behind Martina Grimaldi who took bronze.
Payne told BBC2: "I've always said that open water is about the best person making the right decisions at the right time and I really had to go and feed and I missed the bottle, and from then I really struggled to get back.
"I was probably working a little bit too hard for how I would normally like to take the pace of the race, and then I just had to get back to a good position. I got there but I just didn't have enough at the end unfortunately."
Considering her future in the sport, she said: "I'm not sure how things are going to go. It's going to be my last for a little while - I think I need a bit of a break from doing 10ks. We'll see. It's not the last of me and swimming, anyway.
"Fourth in the world is not too bad but not what I really wanted."