For Haas, good rounds have become commonplace this year on the Champions Tour, where he has won three times in the last 2 1/2 months. In his first round Thursday, he needed to scramble for a 67 while hitting just 11-of-18 greens in regulation.
Barr was steadier over his 18 holes at Prairie Dunes Country Club. He found 12-of-14 fairways and hit 14 greens in regulation, completing his best round in five U.S. Senior Opens.
'You never really feel comfortable at a U.S. Open. I just happened to keep it in play most of the day,' said Barr, who became the first Canadian to win a Champions Tour event when he captured the 2003 Royal Caribbean Golf Classic.
Behind the leaders lie 41 players who are within five shots of the lead.
A deficit like that is nothing to make up after the first day. Just ask defending champion Allen Doyle, who is tied for seventh at 1-under-par 69.
'I don't get excited after Thursdays,' said Doyle, who came from nine shots back on Sunday to win last year. 'This is just the beginning -- [the tournament] is still in its infancy. But you don't want to put yourself behind the eight ball on Thursday or Friday, so today was a good start.'
Doyle isn't the only former champion within striking distance of the lead.
Graham Marsh, the 1997 champion, shares third place with Massy Kuramoto, Mark James and D.A. Weibring at 2-under-par 68.
Marsh, who hasn't won since 1999, finished tied for 68th last week at the Greater Kansas City Golf Classic. He was one of a number of players in their 60s who shot a 71 or better on Thursday.
'I've been playing quite well. It doesn't show it last week, but this is much more my cup of tea, where the premium is not on brute strength,' said Marsh.
'Because I don't care how strong you are, you're not going to advance that ball out of that rough more than a hundred yards. And so brute strength is not what's going to do it here.'
Prairie Dunes is playing short -- at 6,646 yards -- and a lot of the talk Thursday was about the long rough and tough pin placements.
'I think that they have to put in some difficult pins because, other than the rough, the length is not a defense of the course,' said Haas. 'So if you put the ball in the fairway here you can attack a little bit.'
Players may enjoy the short length even more as the weekend approaches and they have to begin thinking about walking four rounds instead of the normal three rounds.
'It's not long, but there's some up hills, you know, through the sand dunes and things like that,' Haas said. 'I don't think anybody's going to say that they can't get around 18 holes here, but it's not dead flat by any means.'
Joining Doyle in seventh place were Bruce Lietzke, Lonnie Nielsen and Bruce Summerhays. Fred Funk made his champions Tour debut and opened with a 2-over 72 to join nine other players in 34th place.