After starting on the 10th at the Celtic Manor Resort, Colsaerts was one over when he reached the 18th, where his booming drive and a gap wedge set him up for an eagle.
From there he carded four birdies in the last nine holes, eventually recording a five-under round of 66 for the clubhouse lead.
He was eventually usurped by Joost Luiten who made seven birdies and a bogey in his 65, claiming the outright lead on six-under par.
Joining Colsaerts on five under is Andrew McArthur, while the trio of Justin Walters, Robert Rock and Sam Walker all sit a shot further back after each man carded 67.
The Ryder Cup quartet of Jamie Donaldson, Lee Westwood, Stephen Gallacher and Thomas Bjorn were all off the pace, with only the Welshman able to break par.
Team Europe captain Paul McGinley had eaten with the foursome the previous evening, and Colsaerts confessed his envy at not being able to join them having been part of the side that pulled off a stunning comeback in Medinah two years ago.
"I saw them having dinner and thought 'I wish I was in that room'," he said.
"It's their adventure and I am not part of it, but once you play in one Ryder Cup, you don't want to miss another one."
On his drive at the 18th, Colsaerts explained that all the elements had been in his favour.
"I thought it was too far right but it's all downhill and downwind and it must have luged down the fairway," he said. "If all the stars align you can hit it a long way."