It is all the more extraordinary because of the hazardous nature of the break, an hour north of Lisbon, where some of the world's biggest waves collapse almost on the shoreline.
Known as "Gmac", McNamara, who was born far from the ocean in Pittsfield, Massachusetts, is one of the world's best-known big-wave riders, taking up surfing after his family moved to Hawaii's North Shore.
He has surfed tsunami waves caused by calving icebergs and ridden some of the world's biggest monsters.
"We'd been invited by the Government of Portugal to Nazare to investigate it for a big-wave competition," he recalls. "There is an underwater canyon 1000ft [305m] deep that runs from the ocean right up to the cliffs. It's like a funnel ... It's unique. The waves break into cliffs 300ft [91m] in height."
Big-wave surfing requires special equipment and a different approach to riding ordinary waves. On November 1 McNamara was equipped with a buoyancy aid and knee braces to protect his joints from the battering a surfer's legs suffer bouncing down the huge wave faces.
"It looks smooth but it's not. It's like bouncing down moguls [on a ski slope]. You hit every ripple in the water."
It is for this reason that in the huge sets the second wave is often preferable. "The first wave grooms the sea bottom and can make the second smoother," McNamara explains.
On November 1, McNamara was not equipped with an emergency air supply to improve his chances of survival if he were to be held down by the surging water after coming off his board. His 1.8m board is equipped with feet straps, like a snowboard, to prevent him being thrown off, and loaded with 5kg of extra weight to increase momentum at the beginning of the ride.
Catching big waves is different to ordinary surfing. The speed of big waves makes it difficult to paddle fast enough ahead of the wave to catch them, so jetskis are used to tow the surfer at speed on to the breaking crest and to recover the surfer at the end of his ride - a dangerous business.
"We went round and I just wasn't feeling it that much," McNamara said. "When we got round to the break it did seem big but I was just going to tow Andrew ... . Then the other guys were saying you've got to go, you've got to go, and - boom! - I was on the rope and on a giant wave.
"Even then I just didn't realise how big it was. So I started and I kept going down and down and the drop seemed like forever. And I thought - wow!
"I started making the bottom turn and I felt the lip hit me. You can see it in the video. Usually I don't have time to look around but you see me look round twice and then I get hit by the white water on the shoulder and it feels like a ton of bricks.
"I've been hit like that before and I'm thinking: I've got to make this. I've never been held down by a wave that size and I've ridden big waves.
"I've seen waves rip a guy's arms off so there is nothing holding them on but skin and I'm thinking this could tear my head off.
"It's only when I got in that I saw the footage. I was in awe. I mean I felt it was a decent size. But you can see it then pops up 10, 20, 30ft [3m to 9m] higher. It just supersizes."
McNamara is not eager to repeat the experience. "I'm not sure I want to ride that peak again."
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