TROON, Scotland (AP) -- Bruce Vaughan shot a level-par 71 to take the halfway lead Friday in the Senior British Open and then headed straight for physiotherapy on his troublesome left knee.

The American has had three partial knee replacements in six operations over the past two years. With the chance of a first-ever senior major in his sights, he was taking no chances as he went off to get some treatment.

"It is my left knee which is why, when it hurts a lot, I don't tend to play so well," he said. "But when I went out today I looked at the scores and I thought that level par would be a pretty good number, so I am pretty pleased with that."

Vaughan has totaled 3-under 139 to lead by one stroke over John Cook of the United States (71). Bernhard Langer (71), Eduardo Romero (73) and Tom Watson (71) are one stroke further back.

Vaughan, a former firefighter who was persuaded to take up golf at age 20 because his job gave him so much spare time, made two bogeys but each time rebounded with a birdie.

Stronger than expected winds led to some high scoring at Royal Troon links on the west coast of Scotland.

Greg Norman, who led the Open Championship on Sunday with nine holes to go but wound up tied for third behind Padraig Harrington, shot a 72 -- including an eagle at the 16th -- to be eight strokes off the pace.

A two-time Masters champion, Langer is one of the favorites having won two titles on his first season in the Senior British Open.

The German, who made a 30-foot birdie putt at the ninth and pitched to within two feet for another at the 15th, said he is playing as well as when he was on the regular tour.

"Sometime even more," Langer said. "I'm not out there to prove anything, just trying to enjoy my game. I know my time is limited, I have only a few more years as a golfer and I'm really having a lot of fun on the Champions Tour."

Romero bogeyed two of the first four holes to lose his share of the overnight lead.

"But my putter is very good," said the burly Argentine, who lost a playoff to Loren Roberts in this championship two years ago at Turnberry. "It gave me a lot of confidence and, if I missed the shots, it's no problem because I'm putting good. I play very relaxed. Still, 1 under and it's good. I'm still happy."

Watson made his first three bogeys of the championship, but said he was lucky to only bogey the par-4 11th where he hit his tee shot into a bush for an unplayable lie.

"I pitched just short of the green and then pitched over the bunker to 12 feet and made that for a bogey 5," he said. "I thought I would have made 8, but I went from 8 to 5 in one hole."

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