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The 34-year-old Sanchez, from the Dominican Republic, took gold at Athens 2004 eight years ago but was not widely considered to be a medal contender this time around.
However, he stunned the rest of the field to come home in 47.63 and edge American Michael Tinsley into second. Favourite Javier Culson placed third.
Sanchez said: 'No-one expected this. A lot of people said I should retire but I stuck with it.
'They'll all be celebrating [in Dominican Republic] now.'
Another Caribbean island celebrating is Grenada, after Kirani James gave them their first-ever Olympic Games medal by storming to victory in the 400m in 43.94 seconds.
USA athlete Jennifer Suhr dethroned Elena Isinbaeva as Pole Vault queen by taking gold in 4.75m.
She won on countback from Cuban Yarisley Silva, while the great Russian, who had been a two-time reigning Olympic Games champion and is the current world record holder, had to settle for bronze.
Australian Sally Pearson impressed in the women's 100m Hurdles in the morning, confirming her status as a gold-medal favourite.
Pearson recorded the fastest time ever in the opening round, a 12.57 effort which eased her through to the semi-finals.
American Lolo Jones was the second fastest qualifier in 12.68 but Brigitte Foster-Hylton from Jamaica, regarded as a contender for a podium place, hit a hurdle and crashed out.
World record holder and 2011 world champion David Rudisha cruised through in the men's 800m, as did Sudan's Abubaker Kaki.
Meanwhile, in the Sailing, Australia earned themselves a gold - and another was virtually assured.
Australia started the 11th full day of competition with just one gold - claimed in the pool by the women's 4 x 100m Freestyle Relay team - but that tally looks set to be tripled after a successful day at Weymouth and Portland.
Tom Slingsby started things off with victory in the men's Laser class.
The 27-year-old came into the regatta as overriding favourite just like he did in Beijing four years ago, where he did not even make the medal race and finished in 22nd place.
Slingsby made amends this week, though, impressing throughout and securing gold in the medal race.
'Who would be feeling better in the world right now? No one,' he said.
'I've worked for 12 years to get here. I put everything I had into it and I've done it. I couldn't be happier.'
Slingsby came into the medal race with a 14-point lead over his nearest rival Pavlos Kontides and kept the Cypriot at a distance.
Kontides does, however, have the consolation of taking Cyprus' first-ever Olympic medal.
Australian success continued with Nathan Outteridge and Iain Jensen all but securing gold in the 49er class. The pair got a first and third to make their lead unassailable heading into the medal race.
Outteridge and Jensen have only got to turn up to compete in Wednesday's medal race - even if they finish last - to clinch the victory.
China's Xu Lijia won the Laser Radial class, while the nation also remained on course to claim all four Table Tennis gold medals after Li Xiaoxia, Ding Ning and Guo Yue booked their place in the women's Team event final.
In the Artistic Gymnastics, Russia's Aliya Mustafina won the Uneven Bars crown ahead of defending champion He Kexin of China and Britain's Beth Tweddle while Brazil's Arthur Nabarrete Zanetti took gold in a dramatic men's Rings final.
South Korea's Yang Hak-Seon took the men's Vault crown.
Giovanni Cernogoraz of Croatia was a dramatic winner of the men's Trap at the final Shooting event of London 2012.
The 29-year-old qualified for the final with the lowest score but missed only once while his rivals faltered.
He ended up in a shoot-off for gold with world champion Massimo Fabbrizi, and it was the Italian who missed first to give Cernogoraz the title.
British dominance continued at the Velodrome with Jason Kenny edging out France's Gregory Bauge to win the men's Sprint final.
The Host Nation's astonishing Games also saw them record an 18th gold in the Equestrian Team Jumping event.
In the women’s Football semi-finals, Japan beat France 2-1 with goals from Yuki Ogimi and Mizuho Sakaguchi, while the USA beat neighbours Canada 4-3 in dramatic fashion.
The game went into extra time at 3-3 and Alex Morgan headed a winner for America with seconds of the added 30 minutes remaining.