Rossi's best lap of 1 minute, 43.1206 seconds (140.131 mph) in the second of three practice sessions held up as the fastest of any NTT IndyCar Series driver on Road America's 4.014-mile permanent road course. But it wasn't easy by any means. In-car cameras showed Rossi battling to maintain control of the No. 27 NAPA AUTO PARTS Honda, nearly sliding off track several times and bouncing into the air after running over curbing.

REV GROUP GRAND PRIX PRESENTED BY AMR: Practice 1 results I Practice 2 resultsI Warmup results I Combined practice results

While he sat atop the leaderboard at the end of the day, Rossi wasn't comfortable with his car when pushing it to the limit.

"Not too happy with the car right now," Rossi admitted. "I mean, I'm pleased obviously there's pace in it, but it's a handful to drive. Hopefully, we can make that a bit better for tomorrow."

Power trailed Rossi on the timesheet by nearly three-tenths of a second. The 2016 Road America race winner was clocked at 1:43.4089 (139.740 mph) in the No. 12 Verizon Team Penske Chevrolet, also in the second practice.

Conversely to Rossi, Power was pleased with his car's handling.

"The car was really good," the 2014 NTT IndyCar Series champion said. "Really happy with it in qualifying trim. Just a matter of massaging on it overnight, get it right in the window. I think we're there."

While the 120 minutes of track time during the day saw just two red-flag stoppages - both in the morning session when Meyer Shank Racing with Arrow SPM's Jack Harvey went off course - numerous drivers dropped wheels off track and kicked up dirt as they tested the limits of their cars and the circuit to wring the most out of a lap. 

Josef Newgarden, winner of three of the first nine races this season and the current points leader, was third quick Friday with a lap of 1:43.4758 (139.650 mph) in the No. 2 REV Group Team Penske Chevrolet. Newgarden, also the defending winner at Road America, leads Rossi by 25 points entering the race weekend.

A pair of rookies rounded out the top five on the combined timesheet. Colton Herta was fourth in the No. 88 GESS Capstone Honda at 1:43.7415 (139.292 mph), with Felix Rosenqvist fifth in the No. 10 NTT DATA Chip Ganassi Racing Honda 1:43.7464 (139.286 mph).

A final practice for the 23 entries is scheduled for noon ET Friday (NBC Sports Goldlivestream). NTT P1 Award knockout qualifying to decide the grid order begins at 4 p.m. Saturday ( NBC Sports Gold livestream, delayed telecast on NBCSN at 5 p.m.).

Live coverage of the 55-lap race starts at noon Sunday on NBC and the Advance Auto Parts INDYCAR Radio Network.