Jenson Button says it was a "bit of a struggle to find my feet" during his first day back in a Formula 1 car on his one-off appearance at the Monaco Grand Prix.
The 2009 world champion is standing in for McLaren's Fernando Alonso as the Spaniard races in the Indianapolis 500.
The 37-year-old Briton, who retired at the end of last season, was 12th quickest in Thursday's practice.
However, he said it was "fun" and "a privilege" to drive an F1 car again.
Button, a winner in Monaco eight years ago, admitted it was taking time to get used to the extra speed of the 2017 cars following rule changes that have made them faster and more demanding.
"It is very different to last year in terms of how late you can brake," said Button, who was one place behind and just 0.035 seconds slower than team-mate Stoffel Vandoorne.
"I'm braking for corners and just feel I'm going to be in the barriers.
"It's strange initially but I'm getting to grips with it. High speed (corners) are so much fun - so much fun. The first part of the Swimming Pool is flat (out on the throttle). I can't remember the last time it was flat. Probably hasn't ever been for me.
"Turn Three heading into Casino is a lot of fun as well. But there are braking areas where I have a lot of work still to do, to get confidence, to brake hard.
"It is the old beginner's thing in F1 - where you brake early, lift off early, turn in too early, accelerate too early and understeer off.
"There is a lot to come and hopefully I can sort myself out on Saturday morning after I have been through all the data."
Button said he had "had a little smile" to himself when he headed out for his first lap in Monaco on Thursday.
"It's a lovely experience to drive these cars," he said. "Have I missed it? No. But when you jump in the car you definitely enjoy the moment you have. I have enjoyed practice, but FP2 was a bit of a struggle to really find my feet with the car because there is a lot of grip with the ultra-soft tyres.
"But I am definitely getting there and I am confident I can improve for Saturday."