It took barely hours for the image to become an instant sporting classic. One moment Jonny Brownlee is striding to victory in the final World Triathlon Series race of the season, about to become world champion for the second time; the next his legs buckle beneath him as if punched by an invisible haymaker.

Then, just as Jonny is stumbling like a drunk into an official, his elder brother Alistair swoops to the rescue, hooking his arm around his shoulder and helping him to cross the line in second place, where Jonny slumps to the floor, floppy and desperately spent from heatstroke.

The pictures of these two brothers in arms brought to mind a hobbling Derek Redmond being helped to finish the men's 400m in the 1992 Olympics by his father or perhaps the American Abbey D'Agostino and Nikki Hamblin of New Zealand aiding each other after a collision during the 5,000m at the Rio Olympics. There were thousands of retweets when the videos were posted on social media and praise from other athletes, such as Jessica Ennis-Hill.

It was a vital intervention. Jonny’s condition was serious enough for him to be rushed to hospital and to miss the podium presentation. Alistair said: “If he’d conked out before the finish line and there wasn’t medical support it could have been really dangerous. It was a natural human reaction to my brother but for anyone I would have done the same thing. I think it’s as close to death as you can be in sport.”

Yet for all the kudos and praise that has - rightly - been directed their way, it would be wrong to sugarcoat what happened on Sunday in Mexico. Alistair was intervening not only to stop his brother falling but also to try to help him win the world title. The mathematics were simple.

Alistair was out of the running for the world title having missed a number of races earlier in the season and he knew Jonny had to win the final race in Cozumel, with the Spaniard Mario Mola finishing no higher than fourth, to take the top prize.

All was going to plan until Jonny pushed too hard in the heat, losing the race to the South African Henri Schoeman and the world championship to Mola. A not entirely sympathetic Alistair said afterwards: "I wish the flipping idiot had just paced it right and won the race. He could have jogged the last two kilometres."

Some might consider that remark a little raw but that is how the brothers are, simultaneously encouraging, chiding, bickering, helping each other to become the greatest triathletes on the planet while also refusing to waste an opportunity to take the piss.

The pair are enormously bright: if it had not been for triathlon, Alistair would have gone to the University of Cambridge and Jonny to Durham, and they like to get one over on each other, physically and intellectually, whenever they can.

Alistair was just as cutting four years ago when Jonny collapsed at the finish again, having won bronze at the London 2012 Olympics. Afterwards Jonny was unable to walk or stand for an hour and was vomiting through his nose. The reaction of Alistair, who had sprinted away to win gold?

“I wasn’t too worried. I’d seen Jonny like that before and I knew he would be OK. Of more concern was that he might have to stand on the Olympic podium wearing only his shorts: his special tracksuit was soaked with sweat and covered in blue vomit.”

Then there were the Games in Rio last month. With the 1500m swim and 40km bike completed, Jonny and Alistair were out on the road on the run in front, battling for Olympic gold and silver. Then Jonny uttered one word to his brother “relax” and Alistair - who interpreted it as a sign of weakness - attacked to retain his title.

Not that this brotherly battle is a one-way street. In their autobiography Swim Bike Run, Jonny admits that when Alistair - then a 20-year-old outsider - was leading the 2008 Beijing Olympics, he was desperately thinking: “This is the Olympics. It’s special. You can’t have my brother winning it.”

When asked about their relationship a couple of years ago, Alistair said: “Jonny would deny it to the core but I think his entire life has been about trying to compete with me. Trying to get level with me and then beat me. There was a time when he wasn’t into triathlon for its own sake; he wanted it because I had it.”

Despite all the one-upmanship they almost always work together across hours and days and months of training each year - on a typical Wednesday, for instance, they might swim for 90 minutes and go for a 75-minute run in the morning before a four-hour bike ride and another 30-minute run in the afternoon - precisely for moments such as in Rio, when they confirmed they were the best two multievent athletes on the planet.

As Alistair says: “Throughout my entire life I’ve had my brother trying to beat me at everything I do. It has been an enormously positive force.”

The feeling is clearly mutual. Jonny tweeted from his hospital bed in Mexico: “Not how I wanted to end the season, but gave it everything. Thanks @AliBrownleetri, your loyalty is incredible.”

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