CHICAGO - World number two Dustin Johnson, making his case for PGA Player of the Year, birdied four of the last five holes Saturday to seize a three-stroke lead after the third round of the BMW Championship.

Reigning US Open champion Johnson sank a 17-foot birdie putt at the 18th hole to complete a four-under par 68 and stand on 18-under 198 for 54 holes at the 7,516-yard Crooked Stick layout in suburban Indianapolis. "I'm seeing the lines pretty well out here, I've got pretty good speed and it's going where I'm looking," Johnson said. "I'm making some putts this week."

The 32-year-old American, who captured his first major title in June at Oakmont, sank three birdie putts in a row starting with a 10-footer at the par-4 14th, then a tap-in on the par-5 15th and lastly a seven-footer at 16. "I struggled with the speed early in the round," Johnson said. "I kept leaving everything short, but I got a handle on it on the back nine and finished really strongly."

England's Paul Casey was second on 201 after a bogey-free 68 with American J.B. Holmes third on 202, four adrift after also shooting 68. A course drenched by rain and stirred by winds made it harsh on leaders and rivals alike. "Really solid. To keep a clean sheet I was really happy," Casey said. "It was pretty ugly. It wasn't my best ball-striking day. The fact I stayed in touch was nice."

Roberto Castro, who shared the 36-hole lead with Johnson, stumbled to a 74 to fall to fourth on 204, one stroke ahead of US compatriot Matt Kuchar and Australia's Adam Scott. Johnson opened with a bogey, needing two shots to escape greenside rough, but answered with a four-foot birdie putt at the second and closed the front nine with a seven-foot birdie putt. He missed the green at the par-3 13th and made bogey, setting the stage for his closing brilliance. "I played pretty solid," Johnson said. "It was pretty tough. The wind was blowing pretty hard. The par-5s played tougher."

Johnson, 11-under on par-5 holes this week, seeks his third title of the year after the US Open and the World Golf Championships Bridgestone invitational. "I'm not going to change anything," Johnson said. "I feel like I've got a good game plan out here."

It would be Johnson's 12th career PGA title and mark the first season in which he has won three events. But Johnson has converted only 2-of-6 54-hole leads into wins, none since the 2013 HSBC Championship. In addition, a victory would put Johnson atop the US PGA playoff season points standings in quest of the $10 million bonus paid out after the Tour Championship in two weeks at Atlanta.

"He's going to be a very tough man to beat tomorrow," Casey said. "He has got the best attitude in golf. When he gets on his game he's maybe the best ball striker in golf, maybe the longest. If he does the same thing tomorrow, there's no catching him." Casey, who will play alongside him in the last group, was in the last group a week ago with a three-stroke lead only to lose to Rory McIlroy.

Casey, a 13-time European Tour winner, won his only US PGA title at the 2009 Houston Open. On Saturday he birdied the par-5 fifth, ninth ancd 15th holes and put his approach inches from the cup at 16 to set up his last birdie.

Scott's 67 matched the day's low round from the field of 69. He closed with back-to-back birdies after ending the front nine with three birdies in a row. "It was nice to make those to give myself a chance to think I can win a golf tournament tomorrow," Scott said. "I've got a lot of chasing to do."

The top 30 players in points after this week advance to the Tour Championship and the five points leaders can win the bonus by simply winning the tournament. Johnson would pace the five at this point with Casey, Scott, top-ranked Aussie Jason Day and American Patrick Reed those players at this stage. Reed and Day each fired 71 Saturday to stand 13 strokes off the pace.