But if both those are shocks, then it is certainly not a surprise to find Lee Westwood in joint second place and Ian Poulter just one further back alongside Tiger Woods.
Couples, champion in 1992, appears a man reborn after switching this season to the seniors tour, where he has won three times in four starts and is an incredible 77 under par for 12 rounds.
America's Presidents Cup captain continued that form with a six-under-par 66 yesterday, his best round in 97 he has played at Augusta National.
"I'm ecstatic about the round," commented Couples. "You're always nervous, because, no matter what age you are, you want to do well. I wasn't expecting too much, to be honest with you, but once I got going I just kept making putts." As he has all year.
Couples would be the oldest ever major champion if he finishes on top on Sunday, as many believe he can – and that itself tells you the magnitude of what Watson is trying to do.
Nine months after being one putt away from winning The Open he is challenging again following a 67 that matched his lowest at Augusta in 119 rounds going all the way back to his debut 40 years ago.
Watson has missed the last seven cuts in the event, crashed out with a worst-ever 83 last year and told Jack Nicklaus on Tuesday that he considered the 7,435-yard lay-out simply too long for him now.
But the words of his son inspired him.
"It's been a wonderful week already," he said. "My son proposed marriage when we played the practice round and his bride-to-be didn't know anything about it.
"Everybody was in on the scam and I think a big part of my success here was having my son on the bag. "He said 'Dad, show me you can still play this golf course.' You know what, I wanted to show him I can still play the golf course."
Watson shares second spot with Westwood, Phil Mickelson and Koreans KJ Choi and YE Yang, for whom this week represents a chance to win a second successive major after overcoming Woods at the US PGA last August. Westwood would have been in the play-off with Watson and Stewart Cink at Turnberry but for three-putting the last and, with another third-place finish at the US PGA a month later, then a second European money list title, he came in as one of the favourites.
He did three-putt twice, but seven birdies made up for those bogeys and he stated: "That's easily the best I've played here.
"That was a 67 that could have been a 64 or anything. It does feel like a golf course that ought to suit me and, over the last few years, I've gradually found a way to plod my way around and feel more comfortable.
"My short game is improving all the time. I'm gradually getting the hang of that after 17 years. And mentally I'm pretty solid, I would say.
"If you get that close (to landing a major) then you know you've proved to yourself and everybody else that you are good enough.
"It's really eradicating the odd mistake, thinking a little bit clearer when it matters. Learning from your mistakes."
Poulter, a winner in America already this year, commented: "It was pretty blustery in parts of the course, so for me to shoot four under par is a great start.
"You have to be patient on this golf course. If it's blowing just make sure you trust it and just keep hitting the shots that you have chosen.
"You can't win this golf tournament on Thursday, but you can certainly put yourself out of it."
Padraig Harrington, Rory McIlroy and Luke Donald have to try to battle back into things after 74s, while Graeme McDowell and Paul Casey had 75s – Casey after reaching three under and then taking eight at the long 15th and six at the last.
At 52, though, Sandy Lyle produced a superb 69 and, at 16, Italian amateur Matteo Manassero, the youngest player in Masters history, did something no British amateur has ever done by breaking par with a 71.
However, Ross Fisher and Simon Dyson struggled to 77s, Oliver Wilson and Chris Wood – a debutant like Dyson – were one worse and 1991 champion Ian Woosnam was next-to-last with an 81.