"I'm definitely not happy with my overall result,'' said Radford. "Everything was going right and tactically I thought it was a very good race. But they just took off those last two laps and they had another few gears to change up to.
"I would be willing to put money on him (Mellouli) to win the gold at the Olympics with the speed he showed in those last two laps. It was something unbelievable. You had to witness it to see how fast that change of speed was.
"It's hard to believe you could swim 7-8km and still have that much speed left in the tank.''
Radford was uncertain if more pace early would have helped his cause.
"May be with more pace early it may have been better for me. But it is always going to be a steady pace and come down to those last 2km and whoever has the extra gear to change up to and they definitely had it.
"I've never been in a race that has changed gears so quickly and so fast. It has opened my eyes that it is the way it is going now, that you have to have that amazing turn of speed over the last 2km.''
While disappointed, Radford said he would be sitting down with his coach Mark Regan to plan his future in the sport.
"There wasn't much left in the tank today. I gave it everything and it wasn't my day. It is all pretty disappointing although looking at the age of those guys I still have plenty more years hopefully to come back and do what those guys did today in four years time and being up there.
"I still have plenty more years to get more experience and perhaps get that feeling from swimming more races at that level so I can learn to change those gears more easily.''
Radford was the leading Oceania swimmer home with compatriot Jonathan Pullon (Waterhole, Auckland) finishing 43rd in 1:53.30.
Yesterday Queensland-based Cara Baker finished 17th and Taranaki's Charlotte Webby 29th in the women's race.
Results, elite men 10km swim, Setubal, Portugal: Oussama Mellouli (TUN) 1:45.18, 1; Richard Weinberger (CAN) 1:45.30, 2; Petar Stoychev (BUL) 1:45.34, 3. Also New Zealanders: Kane Radford 1:47.02, 27; Jonathan Pullon 1:53.30, 43.