Rodrigo Baptista topped the time charts in a mixed-condition opening Pirelli World Challenge GTS practice session at Road America.
The Flying Lizard Motorsports driver reeled off a best lap of 2:20.710 in the 30-minute session, which began in dry conditions before getting increasingly wet.
Baptisa, in the No. 3 Porsche Cayman GT4 Clubsport MR edged out the No. 50 Panoz Avezzano GT of Ian James by 0.888 seconds.
Ernie Francis Jr. in the No. 40 PF Racing Ford Mustang was third quickest, with the No. 04 GMG Racing McLaren 570S GT4 of George Kurtz making it four different manufacturers in the top-four.
Nate Stacy completed the top-five in the No. 14 Flying Lizard Porsche, with Austin Versteeg, making his PWC debut in the No. 13 ANSA Motorsports KTM X-Bow GT4 6th.
Practice 2 is set for 10 a.m. on Friday.
RESULTS: Practice 1