With preferred lies in play at the rain-drenched course in Indiana, the two superstars of golf poured in 16 birdies and an eagle between them.
World number one McIlroy's eight-under 64 put him tied for the lead with US Open champion Webb Simpson, another American Bo Van Pelt and Canadian Graham DeLaet while Tiger ended the day one shot back in tied fifth with Vijay Singh.
Woods could even have led on his own after almost making a hole-in-one at the sixth, his 15th, when his tee-shot caught the edge of the hole.
That was just one of the highlights as he and McIlroy revelled in each other's company, sharing laughs as well as birdies.
"It's a lot of fun," Woods said about playing with McIlroy. "This is the next generation of guys coming out. He hits it great, putts it great, and on top of that, he's just a really nice kid. The game of golf is in great hands with him, and he's here to stay."
Said McIlroy: "I've always enjoyed playing with Tiger, and every time that we're paired up, we seem to have a good time."
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McIlroy, the FedExCup points leader after his win last week at the Deutsche Bank Championship, said he was thrilled with his ball-striking which set up seven birdies and an eagle on 15.
"It was great," he said. "I felt like my iron play was some of the best it's been all year, basically in my whole life.
"I was giving myself a lot of opportunities to attack the pins from the fairways as well. I just have to try and keep doing that for the next three days," said McIlroy who birdied his final two holes to tie the lead at eight-under.
Despite just one shot separating them at the finish, Woods felt McIlroy had played much the better golf.
"I wasn't playing particularly well, but I scored," Woods said. "Rory, on the other hand, played beautifully today [Thursday]. Every single part of his game was working. It all looked good."
World number two Luke Donald also cashed in on the easy scoring conditions, posting a six-under 66 to finish tied seventh with Americans Ryan Moore and Ryan Palmer.
In all, 55 of the 70-man field broke par with England's Justin Rose (-5), Lee Westwood (-4) and Ian Poulter (-4) all well placed.