Russia's Natalia Vorobieva beat Stanka Hristova, of Bulgaria, to win the gold medal in the women's 72kg freestyle.
Five-time world champion Hristova looked in control when she took the lead in the first period, forcing Vorobieva off the mat.
But the triple junior world champion, 21, battled back in style to pin Hristova and earn a shock victory.
Losing semi-finalist Maider Unda, of Spain, and Kazakhstan's repechage winner Guzel Manyurova each won bronze.
Vorobieva said she expected victory after coming through a tough selection process to qualify for London 2012.
"I didn't think about the idea of winning gold because the competition to get in to the Russian team was extremely hard," she said.
"But the moment I learned I was in the team I knew that I would be there standing on the podium getting the gold medal."