Some 40 minutes later, they emerged from a bruising encounter having written themselves into sporting folklore after a contest which ever since has been known as the "Long Count" fight.
An estimated 150,000 packed into Soldier Field in Chicago to witness Dempsey's attempt to regain the world heavyweight title he had surrendered to Tunney a year earlier in Philadelphia.
The champion was out-boxed on that occasion and is later said to have told his wife, 'Honey, I forgot to duck".
At 32, former firefighter Dempsey had learned his trade the hard way, competing in the saloons of mining towns in the West with his brawling style earning him the nickname "the Manassa Mauler".
By contrast 30-year-old Tunney, a veteran of the Marine Corps who served in the First World War – he was known as "the Fighting Marine" – was a cultured boxer and renowned ring tactician.
For six rounds, the two men probed without real incident before in the seventh, Dempsey backed up a left hook to the jaw with a furry of blows which sent Tunney to the canvas for the first time in his career.
However, rather than heading for a neutral corner, the challenger returned to his own, pursued by referee Dave Barry, who crucially did not start the count until the error had been rectified.
Tunney eventually hauled himself up by the ropes with the count on nine, but around 14 seconds after he had gone down, and he somehow survived the remainder of the round before regrouping for the final three to claim a unanimous points victory.