The first match of the Olympic wrestling tournament on Sunday (14 August) produced one of the biggest upsets the Rio Games will likely see.

Unheralded Japanese wrestler Shinobu Ota stunned multiple world champion Hamid Soryan of Iran 5-4 in the opening set of Greco-Roman bouts in the men's 59kg competition in Carioca Arena 2.

Soryan jumped out to a 4-0 lead, but Ota battled back to claim a one-point victory.

Soryan's defeat may lift the chances of Rovshan Bayramov of Azerbaijan of claiming an elusive Olympic gold medal after his two silver medals from Beijing 2008 and London 2012. Bayramov hopes to avoid becoming the second athlete to lose three Olympic wrestling finals without winning one.

Suryan (in red) got the better of Bayramov in the gold medal bout at London 2012 (Photo: Getty Images/Scott Heavey)

Other top contenders in the category include 2015 world champion Ismael Borrero Molina of Cuba and 2013 world champion Yun Won Choi from DPR Korea.

In the 75kg category, Olympic and world champion Roman Vlasov from Russia looks like the man to beat. His main challengers include world championship silver medallist Mark Overgaard Madsen from Denmark. The last Danish wrestler to win Olympic gold or a world title was Hans Heinrich Egeberg at the 1908 world championships.

In another surprise result on Sunday. Daniel Aleksandrov of Bulgaria stunned Olympic silver and four-time world medallist Arsen Julfalakyan of Armenia 3-2 at 75kg.

Shock for Soryan

The loss was the second straight international shock for Soryan, who has won six world titles in addition to gold at the London Games in 2012.

At last year's world championships in Las Vegas, Soryan was flagged for excessive brutality and ruled ineligible for a medal.

After defeating Soryan in Rio, Shinobu Ota stayed on a roll at 59kg, trouncing Kazakh world bronze medallist Almat Kebispayev 6-0.