Colsaerts teed off with Johnson and the left-handed Watson as the first players off the 11th tee and shot a one-under par 69 to finish two shots behind clubhouse leader Phil Mickelson in the weather-interrupted opening round at Merion.
Watson and Johnson both registered one-over-par rounds of 71 in a first round that was halted for three and a half hours due to lightning and heavy rain.
"Being paired with two very good players, it feels good to play good on a big stage like this," said Colsaerts. "It is cool, especially on big tournaments like this, when you see big names up against you."
Colsaerts, 30, was a big hit at last year's Ryder Cup on the outskirts of Chicago, demonstrating his skills and long-driving ability with an impressive debut in Europe's narrow win over the United States.
He was selected by European captain Jose Maria Olazabal as a wildcard pick after winning last year's Volvo World Match Play Championship and immediately showed why he was chosen.
On the first day of the Ryder Cup, he carded eight birdies and an eagle in teaming up with Englishman Lee Westwood to defeat Tiger Woods and Steve Stricker.
Colsaerts missed his last two cuts on the US Tour this season after finishing tied for eighth at the Zurich Classic, but provided a reminder of what he is capable of at Merion on Thursday.
"I don't think I've really peaked anytime during the season," said Colsaerts.
"It's not very far off. When you get to play courses that are pretty demanding, anything can happen, just as is proven today."
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