"That round today was easily 6, 7 under par", Woods said after shooting 70.
Woods implored himself to hole more putts with forecasted Friday night storms expected to further soften conditions at Aronimink.
But consecutive bogeys at his 16th and 17th holes put paid to that notion, before he birdied the par-five ninth after hoisting a five-wood second shot to the heart of the green. McIlroy, who shared the overnight lead with Woods for the first time in their illustrious careers, followed up his first-day 62 with a 69 and is at 9-under.
It was the lowest score for Woods since he shot 61 in the second round of the 2013 Bridgestone Invitational, which he won by seven shots for his 79th title on the PGA Tour and 90th worldwide.
Woods managed an eagle and seven birdies and his only hiccup being a late bogey on his back nine at the Aronimink Golf Course in Philadelphia.
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"I've got to make birdies over the weekend".
Rickie Fowler, Justin Thomas, Rory McIlroy, Hideki Matsuyama and Tiger Woods are all within striking distance after Round 2 and poised to make a move on Saturday at a course that is clearly gettable. Check in for highlights from every tournament plus player interviews, player profiles, tournament previews, swing analysis and all the greatest shots and unbelievable moments.
Moments later, McIlroy hit his tee shot at the 165-yard par-3 fifth to within three feet.
After a 15-minute wait on the tee at the difficult par-3 No. 8-a 246-yarder that has played as the third-hardest par 3 on the PGA Tour since 2011-Woods tugged a 4-iron long and left, leading to an awkward chip and, eventually, his first bogey of the day.
Four early starters including 2016 Masters champion Danny Willett (66) were in a group at 6 under, trailing Tanihara by three. The crowd was much thinner for McIlroy, but those fans saw quite a show. "And I didn't capitalize".