Lahiri rallied from an early bogey for an 8-under-par 63 and has a two-stroke lead over Paul Casey, Tommy Fleetwood and Miguel Angel Jimenez. Thomas Bjorn is alone in fifth at 5-under 66 and six other players, including Jose Maria Olazabal, finished the first round with 67s.
Lahiri started his day on the second nine of the Crans-sur-Sierre Golf Club and had a bogey at No. 13 but recorded his first eagle of the day at the par-5, 597-yard 14th and then added a birdie at 15. He holed his second shot at the par-4 18th for eagle in making make the turn in 31.
He kept his streak going with birdies at Nos. 1 and 2 and pulled clear of the field with birdies at 6 and 9.
"Monday was probably my fourth round in a month," Lahiri said, referring to a knee injury that has kept him off the course. "I came in with no expectations and just wanted to enjoy my golf and that's probably what's happening out there today."
Lahiri led the European Masters after the first round last year but missed the cut the next day.