There was nothing between the two fighters in the first round. Hooper produced the cleaner shots, but it was the volume of punches from Mekhontcev that gave him a 4-2 advantage.
Hooper was the man on the front foot, while the Russian seemed content to wait on the back foot.
The Australian trailed by five as the bell for the third round sounded and he needed to produce something spectacular to save his Olympic campaign.
Hooper tried to repeat his heroics of the last round and let his fists fly in the final round.
However, he found himself outmuscled by Mekhontcev, who did not give him an inch. The Russian unleashed a torrent of blows as Hooper was pinned against the ropes.
Mekhontcev ran out a convincing 19-11 winner.