A draw or win against the Catrachos would leave the U.S.in position to secure a berth with wins against Panama on October 6 at Orlando, Fla., and at last-place Trinidad and Tobago four days later.
He started with a 35-foot birdie putt on No. 10.
"It was a ideal 6-iron for me", said Johnson.
Rahm's round was highlighted by an eagle on the par-5 18th hole.
Masters champion Sergio Garcia shares second after six birdies in his 67, but defending champion Rory McIlroy shot a disappointing 72 which featured three birdies, two bogeys and a double. "This is kind of what, you know - it made me complacent of what I had accomplished all year".
In his final tournament before the President's Cup picks, Phil Mickelson also enjoyed a welcome return to form.
"It was nice to have a solid round", Mickelson said, who had not broken par in the opening round since The Greenbrier in early July.
Branden Grace and Morgan Hoffmann logged the best rounds of the day, each carding a 7-under 64 to ensure both players made the cut of 3-over-par as 79 players remained in contention heading into play Sunday.
The FedEx Cup Playoff field has been trimmed to the Top 100 players this week but only 96 will be in attendance.
"Leish" raced to be three-under par with three birdies in succession from the second to fourth holes and while he dropped a shot at the fifth the current World No. 30 bounced back with birdies at seven and eight. Golf is a game of frustration, and managing that frustration is one of the most important parts of playing well.
"Even though it looked bad, I was making good swings".
His 25-foot birdie attempt on the 14th hole ran about 5 feet by the cup, and he missed that coming back to slow some early momentum. "I feel as though I have managed myself well this year as I haven't played a whole lot of golf while there has been things going on off the course such as another addition to the family", he said.
"I sort of find myself in a similar position this week as I was a year ago, needing a good week to sort of kick-start the next few weeks", said McIlroy. "I played a almost ideal back nine", said Rahm on Saturday after shooting a 31 on his back nine in Round 2. I mean, I think it's a great golf course. Johnson doesn't spend deep analysis over the way he plays.
"I wasn't seeing the shots that I wanted to see".
Rod Pampling is at No. 71 and hasn't made it to the third playoff event since the inaugural year of the FedEx Cup in 2007.
As easy as it looked for Johnson, it was anything but that for Kelly Kraft.
First, at 12:22:32, came Grayson Murray, whose hole-in-one on the 213-yard par-3 8th put him at 3-under on the day and tied for second place at 6-under overall.
All of these guys, however, paled in comparison to Phil Mickelson, who seemed to have a percentage of the Commonwealth of MA following his every shot. His 200 foot approach set him up nicely for a 12-foot putt that curled right to left into the hole.
Ryan Moore, who missed time with a shoulder injury this summer, had four double bogeys in his round of 82.
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