In the classic Pennzoil livery that he and fellow Team Penske legend Rick Mears have made famous over the past 35 years, Castroneves lapped the 2.38-mile Barber Motorsports Park roadcourse in 68.3155sec. That left him just 0.3362 slower than leading teammate Josef Newgarden and 0.16 behind Will Power.

Despite cold and rain wiping out the second three-hour session planned for today, Castroneves was the sole driver to venture out in the wet, turning a 1min24.8887sec on his seventh of eight laps.

The Brazilian, who was lapping the course in preparation for his Indy GP return, said that he was happy to be back in an open-wheel car, after switching his fulltime role to that of Acura Team Penske driver in the IMSA WeatherTech SportsCar Championship.

“I’ve missed it, it was great fun,” he told Motorsport.com. “It was about using muscle memory. So, sure, I am a sportscar driver now, but you cannot take away the open-wheel roots. I have an IndyCar mentality!

“The first run was about learning the new car. It’s very different from the last IndyCar I drove, and it’s very quick. I had to adjust a little bit of my driving style compared with last year.

“I had a few moments out there, to be honest, but I was able to catch it and because of less weight in the back, you can correct and catch it. It’s very slippery on corner entry, but we made some good adjustments and towards the end of the day, we were getting better.

“And those few laps in the wet were fun, too. Chevrolet has made good changes in the software, so it has good driveability. It’s a shame I didn’t get more time out there, but that’s how it is. The rain came, but I know there was more time on the table which we didn’t get a chance to find.”

Despite getting only half a day’s testing in, Castroneves will not get another chance to test on a road course before opening practice for this year’s Grand Prix of Indianapolis. However, despite the undulating Barber track being very different from the flat IMS road course, he said that it had been a useful day for learning and relearning.

“Independent of the track, it was good to understand the new car’s characteristics,” he said, “especially when making changes to it. We tried aerodynamic changes, especially in the front wing, because I needed to see how the car reacts to various adjustments.

“Also, it’s a different steering-wheel dash this year, and you can customize that. So even apart from the big downforce cut, there was a lot to learn.

“The biggest change in downforce, I feel, is felt in the fast turns. You need to think of it like a go-kart, a little more edgy. And that was fun and definitely interesting.

“I wish the test had been longer for us, but still it was a big day, a useful day for me.”