The Northern Irishman, the 2014 Open champion, was five over through six holes on Thursday and seemingly destined to crash out before a back-nine rally.
Poulter teed off in the worst of the conditions early on but he defied the elements and, as the clouds gave way to sunshine mid-morning, he wielded a hot putter only 25 times and mixed four birdies with just one dropped shot to set the early target at three under. Leader Jordan Spieth came out of the rough with his second shot, then made an 18-footer for eagle.
He made bogey and said later, "I nearly screwed up my British Open".
The reigning Australian Open champion and world No.3 fashioned an eventful two-under-par 69 in the wet and wild weather to climb to six-under for the tournament.
As the odd man out of the 77 players to make the cut on five over par, Norris played alongside marker Gregg Pettersen, an assistant professional at Royal Birkdale.
England's Paul Casey, celebrating his 40th birthday by wearing shoes with PC on one heel and 40 on the other, began the day one off the lead at four under but five bogeys saw him go out in 39 and another at the 13th left him two over for the tournament. McIlroy is good enough to card mid-60s rounds at any course he plays, which leads to deceptively good-looking results when he back-doors his way into top-10 finishes. The players that become champions, they get it.
Koepka matched Spieth's 65 and was pleased with how his game fared around the links course, despite having taken a planned four-week break following his US Open victory at Erin Hills last month. The players were hardly more comfortable and probably envious.
Scotland's Richie Ramsay was a shot further back, with Rory McIlroy in a tie for sixth on one under alongside Richard Bland, Austin Connelly and Gary Woodland. I thought a couple would come on the back nine but my patience was rewarded.
Generally he showed control, although one wild shot at the ninth struck a journalist for the Irish Times on the face.
Tommy Fleetwood (+2200), who grew up two miles from Birkdale, has had a meteoric rise to 14th in the world rankings, but being the hometown rooting interest often means a more intense spotlight that can unravel a golfer mentally.
"What we played in previous year as the worst I've ever played in", he said.
While his Irish playing partner was brought up on links golf, Johnson has found the conditions a world away from the SC courses on which he honed his technique. It was his best start in a major since he shot 66 at the Masters a year ago.
"When you ride on the crest of a wave, it's easy to get caught up with those expectations and you start believing them", he said. "I don't think I even felt this way at the '15 Masters after Friday".
"I'm proud of the way I've been able to get things back on the straight and get back to really focusing hard on what I need to do to get the level of golf back that I think I can play", Poulter said.
"I feel great right now", the 23-year-old said.
A 70 percent chance of rain is predicted overnight, according to The Open's weather report, with a 40 percent of "showery outbreaks" through noon.