By Tony Jimenez

TROON, Scotland (Reuters) - The British Open was fortified by the over-40s on Saturday as Henrik Stenson and Phil Mickelson produced a ding-dong tussle to turn golf's oldest major into a gripping two-man slugfest.

It was a case of parry and thrust from the Swede and the American as Royal Troon showed its teeth for the second day running, the third-round winds whipping up to 25mph on the west coast of Scotland and making good scoring extremely tough.

Stenson, 40, began the day one behind his playing partner but ended it one in front after a 68, the joint lowest round of the day, left him on 12-under-par 201.

Mickelson, 46, hoping to become the oldest winner of the tournament in the modern era, returned a 70 for 202.

In third spot, a distant six strokes off the pace, was American Bill Haas (69) with the bearded Andrew Johnston (70) flying the flag for Britain in fourth position.

The popular Johnston, inspired by the fans crying out his nickname 'Beef, Beef' on almost every hole, ended up on 208, one ahead of American JB Holmes (69).

"I know he's not going to back down tomorrow and I'm certainly going to try to not back down either so it should be an exciting afternoon," Stenson told reporters of his thrilling head-to-head duel with Mickelson.

"He's one of the best to play the game in the last 15, 20 years and it's going to be a tough match. I've worked hard these first three days to put myself in this situation and I'm going to try my hardest to finish the job."Stenson, bidding to become the first Scandinavian male to land a major, made his intentions clear when he birdied the opening hole. He then went one ahead of his playing partner after picking up another shot at the fourth.

A bogey at the sixth dented the Swede's hopes before 2013 Open champion Mickelson had a stroke of fortune at Troon's signature eighth hole when his tee shot at the Postage Stamp seemed destined for a trap before spinning back and avoiding the bunker.


Stenson was not so lucky, his effort did find sand and he bogeyed again.

Mickelson then rolled in three nerve-tingling putts at the 10th, 11th and 12th to keep in front before ramming in a 25-foot birdie effort at the 13th to go two clear.

Stenson, though, came battling straight back to level matters at the 14th, delivering a laser-guided approach to six feet and holing out for birdie while his title rival's three-footer for par lipped out.

Seemingly undeterred, Mickelson rallied by coaxing in a seven-foot birdie putt at the 16th before the pendulum swung again a hole later when his playing partner knocked in a 20-footer for birdie to reclaim the lead.

There was yet more drama when Mickelson had an angry exchange with a photographer after finding sand with his approach at the last.

The short-game master, however, conjured some typical late magic to get up and down and remain one behind Stenson.

"I was off today, I didn't have my best stuff," said Mickelson, who opened his campaign on Thursday with the 28th score of 63 in major championship history. "I was a little bit jumpy and my rhythm wasn't very good.

"Today could have been a day that got away from me, instead I shot under-par and kept myself right in there. I'd love to play tomorrow's final round the way I did the first two and give myself a shot."

(Editing by Ken Ferris and Martyn Herman and Pritha Sarkar)