Bradly Sinden was denied in a dramatic men's -68kg taekwondo final in what would have been Team GB's first gold at the Tokyo 2020 Olympic Games.
Sinden, who would have become Britain's first male Olympic taekwondo champion, was narrowly beaten 34-29 by Uzbekistan's Ulugbek Rashitov and had to settle for the silver medal.
The 2019 World Champion, from Doncaster, launched a late comeback attempt and briefly led late in the third period, but Rashitov secured victory in the final moments of the match.
"It was my gold medal to give away -- obviously he is a good fighter, I just made a few mistakes," Sinden told the BBC. "I think I got unlucky with a few things as well but that is taekwondo.
"I thought he was on the back foot. You have to commend him for what he did -- a few mistakes from me, well done to him. You'll see me again in Paris."
Sinden had logged a dominant 20-point victory against Turkey's Hakan Recber in the qualifiers before completing a remarkable comeback to best China's Zhao Shuai 33-25 in the semifinal.
Chelsie Giles won Team GB's first medal of the Games earlier on Sunday, taking home bronze in the women's -52kg judo.
Giles, from Coventry, defeated Switzerland's Fabienne Kocher to secure the win.
The 24-year-old's Games were almost over when she lost to eventual gold medallist Japan's Uta Abe in the quarterfinals. But Giles beat Belgium's Charline van Snick in the repechage to reach the bronze-medal match.
"It feels very special to do it in Japan," Giles said. "It's an amazing arena and the atmosphere was amazing. To do it where Japan started it, it makes it extra special.
"I never underestimate any of my fights, that's where mistakes are made. I go into the fight knowing what they do, and knowing what I'm capable of doing. With my best performance, I know I can beat some of the top players and today, I showed that."
Elsewhere, Andy Murray has withdrawn from the Tokyo 2020 tennis men's singles tournament with a quad strain.
Murray, a two-time Olympic gold-medal winner, said he would instead prioritise the men's doubles tournament alongside Joe Salisbury.
"I am really disappointed at having to withdraw but the medical staff have advised me against playing in both events, so I have made the difficult decision to withdraw from the singles and focus on playing doubles with Joe," Murray said.
Team GB's heartbreak was compiled on Sunday as two-time Olympic champion Jade Jones suffered a shock defeat in the opening round of the taekwondo on Sunday.
Jones, who was one of Britain's brightest gold medal hopes, fell to a stunning defeat against Kimia Alizadeh of the refugee team.
Jones was the -57kg gold medallist in both London and Rio, and was confident of becoming the first British woman to win gold in three consecutive Games.
Elsewhere, Adam Peaty eased into the 100m breaststroke final, clocking a time of 57.63 seconds, as he looks to defend his Olympic title. He recorded the fastest swim of the round once again, while Britain's James Wilby also advanced to the final.
"I had a little peripheral vision," Peaty said after the race. "I knew what I needed to do. 58-low, I knew I could do that, so really, it's about swimming the semi-final, not really proving anything and putting markers down. I know I'll need a lot of energy for tomorrow, so we'll see what I've got.
"Every day has new challenges, new victories, so as long as the sun rises and I'm awake and my eyes are open, anything can happen. Olympic finals are Olympic finals and I'm looking forward to it."
Team GB failed to get onto the medal podium in swimming on Sunday, though, with Max Litchfield narrowly missing out in the men's 400m individual medley final. Aimee Wilmott finished seventh in the women's 400m individual medley.
Germany handed Team GB their first Olympics loss in women's hockey since the London Games on Sunday, while New Zealand powered past Argentina in hot Tokyo conditions.
Team GB took an early lead in the 13th minute when midfielder Sarah Jones netted the ball behind German goalie Julia Sonntag. But Germany hit back in the 24th minute before grabbing a winner just after halftime.
In rowing, Britain reached the men's double sculls and quadruple sculls finals, while Vicky Thornley is into the women's single sculls semifinals.
British rower Angus Groom reflected on the Olympic village afterward and its sustainable cardboard beds.
"Team GB have been very good -- they're sponsored by Dreams and they've given us a really nice mattress topper! It makes it so much more comfortable!"
Britain's Lizzie Deignan finished 11th in the women's road race as Austrian Anna Kiesenhofer caused one of the biggest shocks in Olympic road racing history to take home gold.