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Ainslie, 35, admits it would have been "difficult to top" the feeling of winning at his home Olympics and is keen to move on with new challenges.
"It was a tough decision," Ainslie told BBC Sport.
"I've had a fantastic Olympic career but I want to make it clear that the focus is now on the Americas Cup."
Ainslie won a silver medal at Atlanta 1996, with golds in Sydney, Athens, Beijing and London. He has won more medals than any other sailor, ahead of Denmark's Paul Elvstrom, who has four golds.
He is fourth in Britain's all-time individual medallist's list, behind Sir Chris Hoy (cycling; six gold, one silver), Sir Steve Redgrave (rowing; five gold, one bronze) and Bradley Wiggins (cycling; four gold, one silver, two bronze).
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Ainslie added: "I considered all of the factors - my fitness and the issues with my back, the venue for the next Olympics and the type of boats, but what it really came down to was this opportunity with the Americas Cup."
The sailor envisages skippering his Ben Ainslie Racing AC45 catamaran to glory in the historic competition will be one of the "biggest tests" of his career.
"It's always been a dream