The top three overall speeds came from the first 20-minute session, with Chastain’s average lap speed of 184.268 mph leading the way.
Chastain’s Trackhouse teammate, Daniel Suarez, picked up his first win of the season at Atlanta, coming out on top in a three-way side-by-side photo finish.
Noah Gragson was second quick overall (183.660 mph) in a Ford while Ty Gibbs was third (182.858 mph) in a Toyota.
Chase Briscoe, Gragson’s teammate at Stewart-Haas Racing, was fastest in Group B, but seventh quick overall.
Saturday’s session was plagued by windy conditions, with gusts expected to top 60 mph or more during the afternoon.
Among those drivers who ran at least 10 consecutive laps, Kyle Larson had the fastest average lap speed (180.385 mph) followed by William Byron and Tyler Reddick.
Group B
Briscoe topped the second 20-minute session with an average lap speed of 182.438 mph.
Daytona 500 winner Byron was second (182.346 mph) while his Hendrick Motorsports teammate, Kyle Larson, was third (182.327 mph).
Fellow HMS drivers Alex Bowman and Chase Elliott rounded out the top five.
Todd Gilliland had trouble getting his No. 38 Front Row Motorsports Ford to start and missed much of the session.
Group A
Chastain led the way in the first 20-minnute session with an average lap speed of 184. 268 mph.
Gragson ended up second quick (183.660 mph) and Gibbs was third (182.858 mph).
Tyler Reddick and Denny Hamlin completed the top five.
On just his fourth lap on track, Ryan Preece spun in Turn 2 and backed into the wall, doing extensive damage to his No. 41 Ford.
“I don’t know if it bottomed out or what, but it lost all the grip in (Turns) 1 and 2,” Preece said. “Pretty unexpected.”