Sergio Perez had an "incredible" first day driving his MP4-28, which he reckons has the potential to be a race winner.
The Mexican racer, a favourite with people who like to bet on F1, replaced Jenson Button on Day Two of the Jerez test and covered 81 laps with a best time of 1:19.572. That put him 1.354s behind pace-setter Romain Grosjean.
"It was a day in my new office which was much more different than I was expecting," said Perez.
"By sitting in a different seat you have to adapt yourself to a new position, a different way of holding the steering wheel. Pretty much everything is different. It's like starting from zero.
"But we are going in the right direction, and I'm more happy than I was expecting.
"I still need to sort out some things with my seat, with my cockpit, to be a little bit more comfortable, but even then we still managed an incredible day."
As for the McLaren car, Perez says it is too early to tell how bookmakers like Paddy Power and others will rate it, but he believes it has "good potential.
"I could feel it. We don't know how competitive we are, so all I can say is the car feels quite good.
"We've a lot of work to do over the next five days (his days of testing), so we have to keep our heads down and hopefully we will be in good shape for the start of the season.
"But definitely the mechanical grip on the car is good, as is the aero. It's a very complete car. I'm very happy with it."