Hunter Mahan is in. Adam Scott is out.

Welcome to the ever-changing world of the FedEx Cup points standings.

Mahan shot a 4-under 64 on Saturday in the third round of the Tour Championship, the PGA Tour playoff finale. He tied for the tournament lead at minus-9 and took over the No. 1 spot in the FedEx Cup title chase with one round to play at East Lake Golf Club.

Mahan is tied with Aaron Baddeley, who tied the low round of the tournament with a 6-under 64. Jason Day and K.J. Choi are tied for third at 7 under.

The thought of how the final round could play out and what it means to his FedEx Cup chances are dizzying to Mahan. He entered the Tour Championship a long-shot 21st on the points list. With the top five points leaders, all of whom controlled their fate, down the leader board there is a chance. Or maybe not.

“It’s weird,” Mahan said. “I could still win [the Tour Championship]. I could play flawless golf [Sunday], win by five and finish fifth in FedEx Cup points. I could finish 10th in the FedEx Cup points. I mean I might tee off tomorrow and be 10th. It’s one of those things where it’s like you can’t even worry about it just because you can’t do the math that fast.”

Mahan said that on the 15th hole Saturday he checked the leader board with the projected points standings, and he wasn’t in the top 10. He birdied Nos. 15 and 18 to close his round and take over the top spot.

To win the FedEx Cup title — and its $10 million bonus — Mahan must win the Tour Championship and have Webb Simpson, the points leader entering the event, finish 13th or worse; Dustin Johnson finish fifth or worse; Justin Rose finish in a three-way tie for third or worse; Luke Donald finish third or worse; Matt Kuchar finish in a three-way tie for second or worse; and Brandt Snedeker finish tied for second or worse. All those scenarios are in place.

Simpson is tied for 15th at 1 under after an even-par round. He projects to finish second in the FedEx Cup. If he moves up three spots, Mahan is out again.

“It depends on so many things and so many other people that we’re going to go out there [Sunday] and try to hit the first tee shot in the fairway and play one shot at a time and try to move up into the top 10 or top five for the tournament,” Simpson said.

Scott, who was the projected points leader after the second round, stumbled with a 4-over 74 and is tied for ninth at 4 under.

“So very disappointing,” said Scott, who had two bogeys and a double bogey on the back nine.

Baddeley, who entered the Tour Championship 27th on the points list, at one point was the projected points leader. His round included four consecutive birdies on Nos. 13-16. A bogey on No. 17 cost him a round of 63.

The confusing playoff scenarios don’t much matter to Baddeley.

“My main focus in playing good so I can impress Greg [Norman, International team captain for the Presidents Cup] and to try to win the golf tournament,” said Baddeley, who is 14th in the Presidents Cup standings and needs a captain’s pick to play in the competition in November in his native Australia.

“Those are my main two focuses right now. So much has to go my way to win the FedEx Cup [that] my focus right now is that I want to be on that [Presidents Cup] team.”

Donald, one of the FedEx Cup Five, is tied for fifth at the Tour Championship at 6 under. The others are 14th or worse.

Mahan followed a birdie at No. 8 with an eagle at the par-5 No. 9, making a 25-foot putt after hitting the green in two with an 18-degree hybrid club.

Mahan said he was “shocked” when told in a television interview following the round that he now led the FedEx Cup points standings.

“I honestly didn’t even think that was a possibility,” Mahan said. “... I’m trying to win the Tour Championship. It’s kind of sad for the Tour Championship in a way because it kind of gets lost, and this is really one of the most prestigious tournaments of the year. It’s 30 of the best players in the world playing this week, and the only thing people are concerned about is the FedEx Cup, who’s leading that.”

Mahan is right now. Stay tuned.