Koepka played smart and steady golf throughout the second round and caught fire near the end of the front nine, nailing three straight birdies from the sixth through eighth holes.
A great sand save on the sixth led to an easy two-foot birdie putt. After a birdie on No. 7, Koepka unleashed an excellent approach shot to set up the birdie:
Koepka went just one over from the ninth through 17 holes, but he uncorked a monster 242-yard approach within nine feet of the 18th pin for eagle.
The four-time major winner finished the round at a 13-under, but his nearest challengers aren't going anywhere, though.
Thomas, who started the tournament at 10 under due to the TOUR Championship's new scoring system to determine the FedEx Cup champion, shot a two-under 68 to finish tied for second.
He started things off on the right foot thanks to this 17-foot birdie putt on the third:
Birdies on the sixth and eighth brought Thomas to a three-under on the front nine. However, a disappointing finish dropped him into second after a bogey on the 17th and a par on the 18th following a 5'7" birdie putt miss.
Thomas' golfing was on point, but his caddying efforts left something to be desired while trying to help McIlroy out:
Still, the BMW Championship winner is in contention to be named the FedEx Cup champion for the second time in three seasons thanks to his 12-under mark through two rounds.
As for McIlroy, a rather uneventful 16 holes gave way to back-to-back birdies to finish his Friday performance.
The putter came through on the 17th:
And a fantastic slice on the 18th between trees and over water helped him finish the hole with a simple three-foot birdie putt.
After a six-under 64, Xander Schauffele fell back into fourth with a one-under 69. Bogeys on the eighth, ninth and 11th proved to be the culprits thanks to three drives that landed in two bunkers and then rough.
However, Schauffele's continued dominance at East Lake once again appeared on the final two holes thanks to a birdie following by an eagle on the last:
Schauffele, who is fourth with a 11-under, now has carded 10 straight rounds of 70 or fewer at East Lake:
A couple other golfers made headlines Friday.
Chez Reavie may need a miracle to finish first as he's seven strokes back of Koepka, but this ace certainly helped the cause:
Reavie's 64 proved to be the field's low round and tied Schauffele's opening-round 64 for the tournament's best 18-hole performance.
Paul Casey also caught fire, following a four-under 66 on Thursday with a three-under 67 on Friday to move to nine under for the tournament thanks to his two-under starting position.
Casey's smoking-hot putter was the catalyst, with this 36-footer leading to the day's first birdie:
Casey has shot the most strokes under par this tournament excluding starting strokes. His two-under starting position moved him into solo fifth at nine-under.
Patrick Cantlay is solo sixth at a seven-under, and Reavie, Patrick Reed, Matt Kuchar enter Saturday tied for sixth at six under.
Adam Scott rounded out the top 10 at five under thanks in part to 66- and 71-foot putts:
A lightning delay suspended play for about an hour-and-a-half, although everyone was able to get their rounds in before darkness fell.
Unfortunately, the tournament may be fighting against weather issues throughout the weekend based on National Weather Service reports, with thunderstorms likely on Saturday and Sunday.
Barring any weather-related schedule changes, the third round is scheduled to begin Saturday at 12:15 p.m. ET. Golf Channel will carry television coverage from 1 p.m. to 2:30 p.m. before handing off the baton to NBC until 7 p.m.