(Reuters) - U.S. Open champion Dustin Johnson birdied four of the final five holes to open up a three-shot lead over Paul Casey after the third round at the BMW Championship in Indiana on Saturday.

Johnson delighted fans with another display of long hitting but also displayed a deft touch on slow greens at Crooked Stick in Carmel.

He carded a 68 to post an 18-under-par 198 total, while Englishman Casey also shot 68 to lurk in second place at 15-under, with American J.B. Holmes (68) another shot back.

Johnson has plundered the par-fives, playing them in 11-under, and his length has become an even more valuable weapon on a course softened all week by heavy rain.

"With all the rain we got last night I struggled with the speed (on the greens) early in the round, kept leaving everything a little bit short but got a handle on it the back nine and finished off really nicely," the world number two told Golf Channel.

"I'm seeing the lines really well. The putter is working. It's just going where I'm looking. I feel like I read the greens very well. I'm making some putts this week."

Johnson, using a putter he put in the bag this week, launched his finishing flourish with a 10-foot birdie at the 14th.

He had a tap-in birdie at the par-five 15th and then made it a hat-trick at the 16th, before capping off his day with a 15-foot birdie at the last.

Despite the cushion, Johnson can take nothing for granted.

He has converted only two of his previous six 54-hole leads on the PGA Tour into victory.

"I'm not going to change anything," he said.

"I feel I've got a good game plan out here, be aggressive on flags when I feel like I've got a good club and a good number and other times there are a few holes you just want to make par on and keep going."

Casey is in contention for the second consecutive week. He led going into the final round at the Deutsche Bank Classic, before finishing runner-up to Rory McIlroy.

"To keep a clean sheet, I was very happy with," said Casey, who posted a bogey-free round in "ugly" conditions.

"The fact I stayed in touch is nice, but he's going to be a tough man to beat tomorrow.

"He's got maybe the best attitude in golf. When he gets on his game maybe he's the best ball-striker in golf, maybe the longest, and he showed it today."The BMW Championship is the third of four FedEx Cup playoff events on tour.

The top 30 players on a points list on Sunday advance to next week's Tour Championship in Atlanta.

(Reporting by Andrew Both in Cary, North Carolina; Editing by Peter Rutherford)