Keri-Anne Payne fourth as Risztov wins Olympic open water swim
In a brutal two-hour race of endurance and strength, Risztov held off Haley Anderson of the United States in a desperate sprint to win the title by less than half a second.
Payne had been in the leading pack throughout but despite a late surge could not overhaul Italy's Martina Grimaldi for bronze.
"It just didn't go my way from the start," pre-event favourite Payne told the BBC. "I tried to get up to the lead but they played me at my own game.
"I just made a bad decision on the third lap. I knew we had to feed, but it was a bad decision [to do it then] unfortunately.
"It all went wrong as I needed to work pretty hard to get back where I needed to be."
Payne, silver medallist in Beijing four years ago and reigning world champion, has endured a difficult build-up to these Games and was unable to swim from the front as she usually prefers.
A punishing pace set first by Australia's Mel Gorman and then Risztov prevented her adopting her usual tactics, but she was within three metres of the leaders throughout the first third of the race.
Risztov tried to make a break at the halfway point and the injection of pace split the large lead group of swimmers into three.
Payne was in fifth place, four seconds off the lead, as the race entered the final lap.
Watched on by Prime Minister David Cameron, team-mate Rebecca Adlington and her fiancé David Carry, she clawed her way up one place as the battle for gold was developing a further three metres ahead.
Anderson closed right up on Risztov as the race entered the final 100m into the finish board and the two exhausted swimmers went stroke for stroke in a remarkable finale.
But it was the Hungarian who touched first, stopping the clock at one hour, 57 minutes 38.2 seconds.
It was a wonderful crescendo to an absorbing contest played out in the sort of atmosphere that has come to define these London Olympics.
On a scorching afternoon, the British crowds once again packed Hyde Park two days after the Brownlee brothers had taken gold and bronze at the same venue.
Payne was roared around every metre of the 1.7km lap course as swans and ducks flapped away from the flailing arms of the combatants.
This is only the second time the event has ever been held at an Olympics. Following an equalling gripping race in Beijing four years ago, Thursday's engrossing showdown confirmed that it is here to stay.