Olympic rowers spend huge amounts of time perfecting their strategies ahead of races, but sometimes even the best-laid plans go out of the window.

Such was the case at the Paris Olympics on Sunday morning, when the first heat in the men's pairs turned into an unexpected thriller.

Four of the five boats finished within 0.43 seconds and world champions Switzerland ended up in the repechage after they finished behind Spain, New Zealand and Ireland.

"I think we stuck to our plan, and it almost worked out quite well — but yeah, in the end, it didn't," Swiss rower Roman Roeoesli said with a grin, adding that he and Andrin Gulich were looking forward to their second chance on Monday.

Roeoesli revealed details of the planning process.

"We develop (our race plans) over the racing season, I would say, but in the end we focus on ourselves and we just try to find technical points that we have to pay attention to in the race," he said.

Second after the first 500 meters, the Swiss took the lead until the 1500m mark, but they fell away in a frenetic finish in which Irish pair of Ross Corrigan and Nathan Timoney beat them by .02 seconds.

"I suppose we stuck to our plan 95%," Timoney said. "I don't think any race plan is perfect, not every race goes according to plan, but if you can aim for that 100%, you're doing well."

Asked how communication worked between him and Corrigan when they realized automatic qualification was within their grasp, Timoney said it came down to a single syllable.

"It was just the word 'go' — just 'go,'" Timoney continued. "We have a very set and strict race plan throughout the entire race ... (but) when you need to go, you just go."

Perhaps the best-known plan of all in rowing comes from Timoney's teammate, lightweight sculler Paul O'Donovan, who bluntly revealed his process ahead of the 2016 Olympic final in Rio where he and his brother Gary won silver for Ireland.

"Close the eyes and pull like a dog," O'Donovan told Irish broadcaster RTE, a statement that became an instant internet sensation.