Running the softer alternate tyres, the Swede dropped a lap of 1m00.3390s around the 1.8-mile, 14-turn temporary street circuit to head the leaderboard in the final segment of the session.
The lap from Rosenqvist, who joined MSR from Arrow McLaren over the off-season, put him 0.4722s ahead of former team-mate Pato O’Ward.
Deployed on a trial basis in St. Petersburg, the field was split into two groups for the opening 75-minute practice.
The format featured a 20-minute “All Cars” session to begin the session, then following that, each group – split by odd and even numbered pit boxes – received two 10-minute segments on an alternating basis. Similar to qualifying, the clock for segment one and two stopped for the first red flag condition of each practice segment.
Chip Ganassi Racing’s Marcus Armstrong ended up third, 0.4792s behind.
Will Power, who was running in the penultimate segment opposite of Rosenqvist, O’Ward and Armstrong, ended up fourth for Team Penske (+0.5019s).
Rinus Veekay also presented an early encouraging showing for Ed Carpenter Racing in fifth (+0.5458s) as Scott McLaughlin (Team Penske), Romain Grosjean (Juncos Hollinger Racing) and Colton Herta (Andretti Global) completed spots sixth to eighth.
Reigning and two-time IndyCar Series champion Alex Palou put Chip Ganassi Racing machine ninth while Callum Ilott, substituting for the injured David Malukas, completed the top 10 in a cameo outing for Arrow McLaren.
There were three red flags throughout the entirety of the practice, all of which came in Turn 10.
The first incident came with roughly seven minutes remaining during the second segment when Ganassi rookie Kyffin Simpson locked up and ended up stranded in the runoff.
Less than five minutes later, Pietro Fittipaldi (Rahal Letterman Lanigan Racing) endured a similar fate in Turn 10 to bring out a second red flag.
The last red flag came out for Santino Ferrucci for also blowing the corner in the final two minutes of practice, which was able to resume and finish under green conditions moments later.