Rory McIlroy must win and hope that Dustin Johnson finishes no better than tied second if he's to secure the FedEx Cup and that's now a possibility after he posted a bogey free, four under 66 to get to within two shots of the the lead in the Tour Championship.

For a while it looked as though McIlroy was in for another frustrating day and Johnson was set to cruise into a comfortable lead.

But the tables were turned as the faltering American ended up signing for a 69 featuring six birdies, three bogeys and a double bogey to fall into a share of the lead with Kevin Chappell (66) on eight under par with McIlroy tied for third with Ryan Moore, just two behind, after they shot 66s. Scores

Johnson was three under for the day after 12 holes but he bogeyed the 14th and 15th and then followed a birdie two at the 16th with a clumsy double bogey at the 17th, where Chappell birdied to go into the lead.

After a huge drive, Johnson got up and down from sand short of the par-five 18th green and birdied from three feet to get back on terms with Chappell. The problem is that he's now got McIlroy breathing down his neck at East Lake. 

The Co Down man was already in the clubhouse on six under after a round that did not get going until he birdied the eighth.

Once he made a five footer there, he was off and running and made three more birdies coming home.

With Johnson faltering late in the day, McIlroy knows he has a chance of winning if he can post another round in the mid 60s.

As for the $10m FedEx Cup bonus, nothing but a win will do but he also needs Johnson to finish no better than tied second. 

"That birdie on eight was big," McIlroy said. "I was getting frustrated with myself. I had plenty of chances on that front nine and I was hitting good putts but just burning the edges.

"It was nice to get that one and then I’ve played the back nine well this week - take the front nine out of it and I’d probably be leading the golf tournament, I played well today, more solid, hit a lot of better shots. Did some work on the range last night which I needed to do and I was much better."

Looking ahead to Sunday and four shots covering the top six, he said: "I need to go out and win this golf tournament. That’s what I need to focus on. And if I can do that. someone needs to tie with DJ for me to win [the FedEx Cup].

"I’ve been close a couple of times in my career. It would be nice obviously to win. All I can do now is focus on myself, try to post another good number and even if DJ finishes second on his own, he equally deserves this FedEx Cup.

"He’s had a phenomenal year — way better than I have had. But that’s the beauty of these playoffs. You can come from behind and give yourself a shot, which I have tomorrow.