Montgomerie shot his second straight 5-under 68 on Friday -- his 43rd birthday -- for a two-shot lead after two rounds of the Johnnie Walker Championship.
Paul Casey, with a 71, was second with four players another shot back in third. Montgomerie was at 10-under 136 at the par-73 Centenary Course.
Tied for third were Wales Open winner Robert Karlsson (68), Marcus Fraser (68), Soren Hansen (70) and Andrew Marshall, who had a day's best round at 67.
Montgomerie double-bogeyed the final hole at the U.S. Open when he would have won his first major title with a par.
Montgomerie could have led by more Friday, but three-putted from 20 feet for par on his final hole.
'Two good shots into the ninth and from 20 feet I three-putted,' Montgomerie said. 'I don't know what's going on -- it's not good at all. If I can't two putt from 20 feet, there's something wrong.'
On Thursday, Montgomerie bogeyed the last hole, after taking three shots to get down from a bunker.
'Did it tarnish the U.S. Open? Of course it did,' he said of his final hole woes. 'Did it tarnish my rounds yesterday and today? Yes.'
But Montgomerie was still happy with his score.
'That's good around here,' Montgomerie said. 'The course is playing quite tough and the wind is up. I look forward to trying to shoot two more 68s at the weekend.'
Montgomerie, the chairman of the tournament, made six birdies against only one bogey.
'That's good. I'm making very few mistakes, making a few more birdies and making a few more putts,' Montgomerie said. 'But not to hole for birdie at the last is very disappointing.'
There was a funny moment at the 18th when Montgomerie asked the gallery to be quiet: 'No, you be quiet,' came a response -- from a 2-year-old boy.
'It's the first time anyone has answered me back,' Montgomerie said with a laugh.