During the match at Tehran's 100,000-seater Azadi Stadium there were not the usual outbursts of joy and the chanting of football hymns.
Instead, religious authorities had asked Iranian football fans to raise clenched fists and chant "Ya Hussein", the slogan of tribute to a Shia martyr, during the match.
At half time, singers performed solemn religious songs on the pitch.
There was screaming, however, when Iran’s Sardar Azmoun's scored the only – and winning - goal of the match, in the 25th minute.
But during the rest of the match the fans were quieter than normal.
Shias mourn Hussein
Tasu'a is the time that Shia Muslims mourn the start of the epic battle of Karbala in 680 AD according to the Christian calendar, when Mohammed's grandson, Imam Hussein, was killed together with 72 of his followers and members of his family.
In the whole of the country, during the two days of mourning, men in Iran gather in long processions and beat their chests to the rhythm of a drum.
Some flagellate their backs with chains as onlookers watch them. Meanwhile, radio and television channels only play sombre mourning music. Cinemas and theatres are closed, as are government offices and shops.
The South Koreans were not happy.
Some complained that Iranian fans had thrown bottles at them and one player compared Iran with a “giant jail.”
Football rivalry between Iran and South Korea is strong and both countries strive to be Asia’s best.
After winning this match, Iran finds itself at the top in group A with for wins and one draw, followed by Uzbekistan, with South Korea only in third position.
China, the most populous country in the world, has not managed to translate its huge human potential into football success and is the last in group A with only one draw.