Caroline Masson put together a seven-under round of 65 to pull a shot clear of her rivals at the British Open.
The German finished the day on 11 under at the tournament's halfway point after collecting seven birdies on her second round at Carnoustie.
South Koreans In-Bee Park, who carded an eight-under 64, and first-round leader Meena Lee are in joint-second on 10 under.
Compatriot Se Ri Pak carded a 64 as part of a group two shots further back.
Masson, who won the Ladies' European Tour School in Spain in December, is still searching for her first win on the circuit and was clearly surprised to find herself clear at the top of the leaderboard.
"This is absolutely awesome," said the 22-year-old.
"It is quite unbelievable. I didn't expect to lead but it has been two great days. I love links golf, but I had never played Carnoustie before, but I had seen it on TV."
Lee had led by a shot from American Brittany Lincicome after the opening round of the tournament, but both players fell back.
Lee could have been further off the pace, but salvaged a three-under 69 from a disappointing day with birdies at 17 and 18.
On a day in which none of the top 46 players in the field strayed over par, Lincicome came closer than many with a round of 71.
Defending champion Yani Tseng is poised four shots off Masson after she picked up another two shots on her way to the clubhouse following an eagle three at the 14th.