The Scot, who has won two senior majors this season, triumphed by six strokes last year and he took little time to get into the red again, birdieing the opening hole of the Duke's Course before picking up further shots on the third and 10th, before holing an incredible snaking 25ft birdie putt down the two-tiered green on the 14th hole.
"It was difficult out there today," he said, referring to the tricky swirling winds on the tree-lined course, which allowed only nine players to break par. "You stand over the shot, and one minute it is one club and then another club the next minute. So, to go round bogey-free is very good. It was hard to get it close to the hole, and I'm delighted to get in with a 68."
The 2010 European Ryder Cup captain and Rusnak are two shots clear of a group of four players, which included Montgomerie's playing partner Cesar Monasterio, winner of last week's English Senior Open, who began his bid for back-to-back victories with a 70.
Montgomerie, who won the US Senior PGA and US Senior Open earlier this season, started with consecutive rounds of 68 last year, and he is now 14 under par for his four tournament rounds at Woburn, with his consistency off the tee a crucial factor.
"I drove the ball particularly well, and knowing the course as I do from last year made it easier, but the conditions made scoring higher than you'd expect," he said. "Driving well is the key to success round here. You have to hit the fairways, and not just that but the right side of the fairways, and I did that today."
Carl Mason holds the record as the quickest player to win four times on the Senior Tour, doing so in just 11 events, but if Montgomerie retains his title tomorrow, it would be his fourth win in just nine appearances.