Keselowski set the pace early in the 50-minute session at Phoenix Raceway, with a lap at 135.384 mph that stood up the entire session.
Larson was close behind with an average single-lap speed of 135.292 mph. Larson was one of the few drivers to test out the area of the track covered with resin, used to improve grip in the corners.
“I haven’t seen anyone run up there consistently yet,” crew chief Cliff Daniels told Larson late in the session. “I don’t know how you feel right now, but you’re better than the 12 on (new tires).”
Matt DiBenedetto ended up third-fastest (135.109 mph), Christopher Bell was fourth (135.084 mph) and reigning series champion Chase Elliott rounded out the top-five (135.018 mph).
Completing the top-10 were William Byron, Blaney, Denny Hamlin, Alex Bowman and Tyler Reddick.
Martin Truex Jr. was the only Championship 4 driver who did not end up in the top-10. His average lap speed ended up 18th-fastest.
Blaney had the fastest average lap speed among all the drivers who ran at least 10 consecutive laps (132.596 mph). The rest of the top-five: Larson, Elliott, Keselowski and Kevin Harvick.
Early in the session, the practice was placed under caution when a fire erupted in B.J. McLeod’s car. He quickly exited the car and was unhurt.