* Thomas benefits from unique scoring system
* Shoots even-par 70 at East Lake
* Tied with Koepka and Schauffele (Adds quotes details)
Aug 22 (Reuters) - Justin Thomas missed two short putts in a mediocre even-par 70 on the first day at the Tour Championship on Thursday but ended the round tied for the lead with fellow Americans Brooks Koepka and Xander Schauffele.
Thomas was grateful for the unique staggered scoring system, which afforded him a headstart at 10-under-par thanks to his top seeding as FedEx Cup points leader entering the season-ending event at East Lake in Atlanta.
Third seed Koepka birdied three of the last four holes to card 67 and Schauffele had a 64, the day's best round by two strokes.
Americans Thomas, Koepka and Schauffele led the 30-man field by one stroke from Northern Irishman Rory McIlroy (66).
The winner on Sunday will receive $15 million, the sport's biggest payout.
Thomas was grateful for his lofty position after finding the punishing rough several times, and also pushing his tee shot into water for double-bogey on the par-three 15th.
Add in a couple of missed three-footers and it could have been a lot worse.
The runaway winner of last Sunday's BMW Championship said he had paid a price for his slight inaccuracy off the tee, even if his misses were not very far off line.
"The difference between fairway and rough out here ... is so huge," he said.
"To come in the house at 10-under feels pretty good. I thought I played better than I scored.
"I played a lot of good golf the last five or six holes other than that dumb mistake on 15.
"I still can't believe those two short putts. I hit them right where I wanted."
World number one Koepka said it had been "weird" teeing off three shots from the lead on the first day, but thinks the tournament will feel more normal from now on.
He found some confidence on the greens by sinking a 25-footer at the fourth and then almost holed his approach shot at the next for a tap-in birdie.
"I don't know how it missed," he joked.
Joint leader Schauffele won the 2017 Tour Championship by shooting the best score over 72 holes under the regular scoring system.
This time he might need the best score and then some, after starting as the eighth seed, six strokes behind Thomas.
Schauffele could have enjoyed the sole lead, but he missed a five-foot birdie at the last, which left him fired up.
"As weird as it sounds, I'm happy I missed that last putt so I'm not super satisfied with the round," he said.
"I'll go out tomorrow with a little bit of vengeance."
McIlroy also missed a short birdie putt at the last, leaving himself with a five-foot comeback attempt that was almost as long as the first. He made it.
(Reporting by Andrew Both in Cary, North Carolina, editing by Ed Osmond/Greg Stutchbury)