At 12 under, the pair is one shot ahead of last year’s runner-up, England’s Paul Casey, entering the event's traditional Monday finish.
Thomas reeled off another of the low rounds he has become known for of late, an 8 under par 63 improving him from his overnight standing of 15th to take the clubhouse lead before Leishman joined him just under an hour later.
Leishman’s bogey-free round of 65 has not only moved him into a share of the lead but has opened up all sorts of possibilities in regards to the FedEx Cup outcome and has him projected to move to 4th in the overall standings.
A win tomorrow for Leishman, would also move him inside the top 20 on the world ranking, his highest ever standing in world golf.
Also part of the chasing pack are Jordan Spieth, who is just two behind, and World Number One, Dustin Johnson, whose round of 66 today moved him within three of the lead.
“All the chances you should make I did and rolled in a couple of long ones as well which turned a really good round into a great round,” said Leishman immediately after his round.
“I’m where you want to be. When I came here on Tuesday, that was the plan to try and give myself a chance to win and I have done that but there is still a lot of golf between now and when they give out the trophy.”
Leishman has always played at least some of the FedEx Cup Playoffs and today talked about what winning the FedEx Cup would mean to him.
“I mean, myself, I would look at winning the FedExCup like winning a major. I think it's become that big. I think a lot of players, you know, because it's a season-long race, I feel like it's got a lot of meaning.”
Jason Day improved further on day three to be in a share of 44th but after his horror start on Thursday, he is perhaps relieved just to be still in the event. Day is now a projected 31st in the FedEx Cup rankings and will comfortably make the BMW Championship in two weeks’ time before looking for a place in the field of 30 for the Tour Championship in Atlanta.
Rod Pampling finished 77th this week and is now all but out of the Playoffs as is Adam Scott who missed the cut this week.
Australians guaranteed starts at the BMW therefore are Leishman, Day and Cameron Smith who, despite missing the cut this week, has enough points to head to Chicago.