Spain's Ignacio Garrido hit a birdie at the 17th to edge ahead of the pack after two rounds of the Johnnie Walker Championship at soggy Gleneagles.
He carded a 69 to go with Thursday's 67 and sits at eight under par, one shot ahead of a quintet including English pair Kenneth Ferrie and Mark Foster.
Scotland's Marc Warren leads the host nation's players, on five under par.
Defending champion Edoardo Molinari hit a second-round 70, making the level-par cut by the tightest of margins.
Garrido, a member of Europe's 1997 Ryder Cup team, clinched his fifth birdie of the day at the par three 17th.
Ranked 197th, Garrido has little room for error in the remaining two rounds with no fewer than 26 players within five strokes of him at the halfway mark of the tournament.
The 39-year-old is refusing to get carried away, though, on the course that will host the Ryder Cup in 2014.
"It's very easy to do that in this game and one of the things you learn is to just stay in the present," he said.
"It's been one of the key things these two days, that I've managed to hit one shot at a time and not look too far ahead.
"It's not an easy course and it doesn't give you many birdies. But my game's been solid."
With a round of 71, Foster, the overnight leader after Thursday, took five shots more than on the previous day but remains very much in contention, along with Ferrie, Denmark's Thomas Bjorn, Ireland's Peter Lawrie and Italy's Lorenzo Gagli.
England's Ross Fisher (one over) and Jose Maria Olazabal (four over) - Europe's captain at the Ryder Cup in Medinah next year - were among the big names to miss out.
A total of 51 players were forced to finish their first rounds early on Friday after thick fog had wiped out 160 minutes of play on the opening day.
Bjorn dropped just one shot in his second round, on the 15th hole - his sixth hole of the day - but three birdies around the turn and a fourth at the long par-four seventh hole propelled him up the leaderboard.
Ferrie mirrored Bjorn's scoring to move into contention for a first win since 2005, with his recent switch to a belly putter paying dividends.
Gagli - a graduate from the second-tier Challenge Tour - shot 68 to continue a strong recent run in which he finished third in the Scottish Open, eighth in the Irish Open and, last week, fifth in the Czech Open.
Montgomerie shot a 73 for to put him one under, keeping the tournament chairman and 2010 Ryder Cup captain in his home event for the weekend.
But Olazabal, Montgomerie's successor, picked up a two-shot penalty on the second for hitting the wrong ball, making a double-bogey seven.
Two rounds of 74 meant that he missed the cut.