Lahiri, playing his fourth week on the trot, had put himself in some kind of danger at two-over 74 in the first round.
But he went through a first nine on the back stretch of Muirfield Village at Jack Nicklaus' event. He had four birdies against just one bogey to turn in three-under, at which point he was one-under for the tournament.
On the second nine, he went into the water on the second and then into the bunker and ended with a double.
At even par Lahiri is tied 40th.
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It was a gritty performance from the 29-year-old Indian, who has been having trouble on the greens. But with this round of 70 that brought his mental strength to the fore, Lahiri could turn the fortunes with a good result over the weekend.
As the cut fell at three-over among the big names missing the weekend were world No 1 Dustin Johnson (78-74), Billy Horschel (78-71), Retief Goosen (77-72) and Luke Donald (72- 76).
Daniel Summerhays (69) was lying second, but was five shots behind at nine-under. Another shot behind him was Rickie Fowler (66) at eight-under. Then Jordan Spieth (72), Jamie Lovemark (69) and Justin Thomas (71) were tied for fourth spot.