That there has been a changing of the guard in British distance freestyle swimming was obvious the moment Daniel Fogg beat David Davies, the Olympic silver and bronze medal winner, at the London 2012 trials in March.

That rivalry will reach its head this weekend when the two go head to head off the Portuguese coast over 10km at the FINA Olympic qualifier to see who will race in The Serpentine at the Games this summer. 

The top ten men at world titles in Shanghai last year went through at the helm, while those nations who failed to make the grade face their last chance of getting one man and one woman in the London race. Among the new challengers in open water is 1500m Olympic champion Oussama Mellouli (TUN), the California-based charge of coach Dave Salo.

Like Mellouli, Fogg is relatively new to open water. If he is the man who qualifies for a home Games 10km, you might see Davies approach his former training partner on the pontoon in Portugal and ask for a ton of kit back.

“When I first met him he was a student and very much trying to work his way up the ladder,” Davies said. “He was a minute slower than me at that stage in the 1,500 metres. He didn’t have any of the gear, no racing suits, sponsorships. I’d give him a couple of suits from my sponsor but now he’s beating me I might ask for them back.”

Truth be known, there is no animosity. “There’s no bitterness - unless he punches me on the start line!" says Davies. "Dan is one of my closest friends in swimming and one of my best mates,” Davies said. “We’re very close, we talk often and we’ve got a lot of things in common. I imagine it’s going to be a bit bizarre: we’ll room together but one will go [to the London marathon] and one won’t be there.”

At the trials in March at the London Aquatics Centre, Fogg swam inside 15 minutes over 1,500 metres for the first time. The waters where the Sado River meets the Atlantic in Setubal, south of Lisbon, is a very different place. Davies learnt how different the open water event can be back in Beijing when, disorientated in what was only his third 10km race, he veered off course on the home stretch and Maarten van der Weijden, the Dutchman, stole past him on a straight course to gold.

“I was the best swimmer but not the best open water swimmer,” Davies said. “At that point I was a virtual amateur. I was the third race that I’d ever done and I won an Olympic medal. I was quite proud of it. It’s not going to haunt me in my 40s that I should have won gold."

Though the Beijing 2008 marathon went down as a first - and was the longest swimming race in Olympic history - it was not the inaugural open water test at the Olympic Games, the London 1908 pool the first fully enclosed tank since it all began in 1896.

Back then, Alfred Hajos, of Hungary, caked himself in "half an inch of grease" to brave the cold waters for a 1500m battle he won by almost 3 minutes in the Bay of Pireaeus. Four years on in Paris there were races of 1500 and 4000m, both won by Britain's John Arthur Jarvis, who was described by a contemporary as having "fat all over, which literally hangs in parts. His breasts fall like a woman's but he has powerful shoulders and tremendous thighs".

Fogg and Davies are somewhat leaner though they share good shoulders and thighs with Jarvis and have shared some of the same conditions that the 1900 champion would have been familiar with, sewage and dead animals in the mix. 

Asked to described the worst of his experience, Davies said: "I've been lucky to be honest. I remember laughing about the dead dog that floated by in one race. Alan Bircher [former Britain teammate] told me about the time they had to start a race early because they didn't want to wake up the crocodiles on the river bank."  

The Serpentine is the closest thing you can get to pool conditions, the waters cleansed for the occasion and likely to be around 18C or more during the Games.

After the Games Beijing, Davies moved to Loughborough and switched coach to Kevin Renshaw, leaving Cardiff and his lifelong swimming mentor, Dave Haller. The challenge was just what Fogg, now 24, needed and came just at the right time - but for Davies it proved too much given the backlog of work in his bones.

“Overtraining, chronic fatigue, burnout call it what you want,” he said, looking back at a time when retirement appeared to be the only way out. “There was a point at which i thought it might be me. I did all the blood tests, saw doctors, considered nutrition - but everything was fine. My diet was fine. I was just knackered, basically.” There was no ill-feeling toward Renshaw when Davies packed up and went home to Wales.

Rest and a gentle return over two years were what Haller prescribed when his charge returned to his native Cardiff. “He [Haller] can read me very well. Swimming is hard and distance swimming is even harder,” Davies said. “I’m not old at 27 but I’ve been doing this for a long time: ten years of doing 75km a week is bound to take a toll eventually. I’ve felt guilty about not being able to do things I used to do but you can’t compare yourself to the 19-year-old you were. It’s almost like you’re looking at two different athletes.”

Whoever wins the ticket this weekend, expect to see both men at The Serpentine this summer, one cheering on the other. “It’s a fantastic setting. The ideal scenario,” Davies said. “I’m praying for a sunny day. The whole thing could be a really good image of the Games.”

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