EDMOND, Okla. -- Colin Montgomerie thought it would be tough to keep his advantage from the first round at the U.S. Senior Open, and it was.
The Scotsman shot an even-par 71 on Friday and leads Scott Dunlap by one stroke after the second day of the tournament.
Montgomerie opened with a 65 on Thursday, and then held off the field in mid-90s heat and high humidity. He is at 6 under after two rounds.
''It's not easy to repeat a good round as yesterday's was, but this was as good, to be honest,'' he said. ''Seventy-one out there was a good effort. Breezy conditions, windier than it was yesterday, warmer and with the pack coming at you - it's never easy.''
Dunlap had a 69 in the first round and a 68 in the second to get to 5 under. He was at 2 under after starting on the back nine, and then went back to the front nine and birdied Nos. 2, 5 and 7 on his way to a 32.
''I have been particularly good with the putter this week,'' Dunlap said. ''Been tidy. I have made a couple, made all my short ones. You know, those will be big in any tournament, for sure, but especially a major.''
Dunlap has one bogey in 36 holes on a course regarded as one of the most difficult in the United States.
''Take my hat off to that,'' said Bernhard Langer, who is tied for third. ''That's good playing. It's all I can say. It's very difficult not to make bogies here. You can bogey every single hole. There's no easy hole out here, and that's very good ball striking or incredible scrambling, whatever he does.''
Langer, who leads the Champions Tour with three wins this season, moved into a tie for the lead with a 3-under 32 on the front nine, and then grabbed the advantage with a birdie on 14. He had bogeys on 15 and 16, both on close misses, and then bogeyed 18 to finish at 4 under, tied with Gene Sauers.
''The score was pretty decent, but the round was very up and down,'' Langer said. ''Short game was good, really good. Made some good putts and good saves. Long game wasn't quite up to par. Missed a lot of fairways today, and then you're behind the 8-ball and trying to struggle and scramble.''
Sauers shot 69 each of the first two days, but he was much happier with his Friday score given the heat, wind and firmer fairways. He scored an eagle on No. 7.
''I was pretty bad yesterday,'' he said. ''I got a lot of sleep. Went to Chick-Fil-A and went right to bed. Feel better today. Today went well.''
Marco Dawson shot a 66 on the opening day, good for second place, but had a 76 on Friday and fell to even.
''It was just awful,'' he said. ''You land the ball in the fairway, goes in the rough. Yesterday I hit balls that landed in the rough that ended up in the fairway. So today I was hitting it more where I was looking, but they didn't end up in the right spot. I'm not sure exactly how to play now.''
Lance Ten Broeck took advantage of an early tee time and fired a 69 to go to 3 under. Jeff Sluman also had a 69, including an eagle on the par-5 seventh hole, and is at 2 under. Vijay Singh was at 5 under before shooting a 39 on the back nine to fall to 2 under.
Scott Verplank struggled on his home course and shot a 75 for the second straight day to miss the cut. He said he hasn't played this course so poorly in 25 years.
''My golf game is not very good, and I'm going to have to figure out a way to play if I want to keep playing, because whatever I'm doing isn't working,'' he said.