Gentlemen, start your scenarios. Just as it was wrapping up Saturday afternoon, hustling so it could clear network air space in favor of the spectacle that is Notre Dame-Duke football, this Tour Championship got real.

Dustin Johnson had threatened to make everything moot at East Lake. He was cruising to a tournament victory that would assure him the FedEx Cup and its $10 million bonus while everyone else played for diamond-encrusted scraps. Then he hit the 17th hole. And it hit back. By the time Johnson put down his cleaver on the par 4, having butchered it well and truly, he had a double bogey in the books and a lot more players invading his space on the leaderboard.

A Sunday of greater possibilities was born. Now Johnson has company at the lead at 8 under, one Kevin Chappell, who shot a 68 to Johnson’s 69 Saturday and has played this week as if he would rather take a job as a telemarketer than make a bogey.

And welcome to the party Mr. McIlroy. Rory McIlroy, remember him? He put up the low round of the day — a 66 — shared by many (six others went 4 under) but put to better use by none. He snaked up the big board and positioned himself at 6 under for the event, just two back of Johnson.

In all, there are six players within five shots of the Johnson/Chappell lead. And two of those — McIlroy and Paul Casey (3 under) — have a legit chance of denying Johnson the FedEx Cup riches.

Chappell doesn’t fit so much into that category. He has his own quest to wage: Trying to win a tournament for the first time as a six-year PGA Tour pro.

Finishing second out here can be a beautiful thing. Just for the three events in which he was runner-up this season, Chappell pocketed $2.43 million. Such a year, he said, “has taken me to places I’ve never been before.”

But winning, and climbing over the fearsome Johnson in the process, would be pretty OK, too. “No one believes in me so I got nothing to lose,” Chappell said. “I’m going to go out there and give it all I’ve got, and Dustin’s going to play great golf. He’s been in this situation a lot this year, and it will come down to one or two shots.”

But let’s return to No. 17, shall we?

Until that point, it seemed Chappell was powerless to affect Johnson. Through three rounds Chappell committed just a single bogey, by far the lowest total in the field. “I have played some beautiful golf,” he said, “and I’ve eliminated some of the silly mistakes that tend to haunt me.”

On the 12th hole Saturday, he made his first birdie of the day by chipping in from just off the green (his second chip-in of the tournament), and performed some first-degree fist pumping. The Johnson rolled in a 13-foot birdie putt on top of Chappell and displayed all the emotion of a man crossing an empty street.

But the par 4 17th was the big swing. Finally Johnson found a rough he couldn’t bully. From the left rough, he nicked a tree branch and finished still 60 yards from the pin, in the left rubbish. His third shot found the bunker short of the green. Two putts later, he was the unhappy parent of a double bogey. When Chappell made his 8-foot birdie putt there, it was a three-shot swing and he had the lead solely, briefly.

On the par-5 finishing hole — and a par 5 is but a bit of melon on a toothpick to Johnson — he pumped out the fifth birdie he has made on East Lake’s two par 5s this week and returned to a tie for the lead.

Johnson has won at least one PGA Tour event during each of his nine years on the circuit. He’s gone nuts this year, winning three times, including his first major championship. He is the sure-fire player of the year and one who seems fated to assume the No. 1 ranking in the world.

Might he have the advantage when paired again on Sunday with a player still seeking his first victory?

“I’ve got a lot of confidence going into tomorrow,” Johnson said. “I know I can get it done out there. I’ve won quite a few times out here so that definitely helps.”

And the scenarios are hypothetically back in play. Like the one that would have McIlroy winning the Tour Championship and the FedEx Cup if Johnson finishes in a tie for second or worse Sunday.

“Obviously if I was tied with Kevin on the last hole (for second, with McIlroy already the winner in the clubhouse) I’m probably going to be a little more aggressive,” Johnson said smiling.

Hey, it’s possible, which is more than could have been even contemplated before the weekend.