"It was pretty good considering I hadn't driven those cars before," said Hartley on the eve of his Monza race this weekend.
"It's a bit tricky when you turn up at the track and only have 30 minutes testing and to learn the tyres and the car.
"It was tough but in the race I ran quite quickly and had to learn everything about the car in the race.
"I was quite disappointed with the Sunday race because I started fourth but lost a few places on the start and lost a few more placed mid-race.
"It was just one of those races but as the first race went so well the team [Ocean Racing Technology] was just so happy."
The GP2 car, for all intents and purposes, is not too dissimilar to a modern F1 car and is the last steppingstone to getting into Formula One. An added bonus is that the series is a support class for the F1 circus and so drivers have a chance to showcase their talents in front of F1 teams and principals.
"If you do well in GP2 everyone see's it. A GP2 car is probably quicker than some of the back markers in a Formula One race.
"In Spa we were doing more than 300km/h on the straights, which is similar to an F1 car.
"At Monza we'll be doing upwards of 330km/h so I think it's almost the same as Formula One. And the braking and lap times are pretty comparable as well," said Hartley.
Hartley's chance at mixing with some of the best drivers not to be racing in Formula One came about out of the blue. Although he had tested well with the Ocean team at the end of last year in Abu Dhabi nothing came of it.
He is still not sure how the two drives came about, other than receiving a call offering him a chance to showcase his stuff in the feeder class to F1. Needless to say Hartley did not have to think twice about the offer and was packed and ready to go within hours.
"It's exciting doing Monza this weekend and I've got to know the car a bit better.
"The team didn't have any points so far in the season and my race [Spa] was the first time they had scored any points and they're very happy.
"The team's very professional and I'm very happy to race with them, and we get on well. They just called me up and said they wanted to put me in the car for the last two rounds of the series. It's fantastic," said Hartley.
The Kiwi driver resumes his Formula Renault 3.5 litre duties next weekend at Paul Ricard, France, and should be on fire considering the seat time he's had since the last round at Silverstone in August.
"It's been a tough year in Formula Renault and I had a whole new team for Silverstone.
"It was a big mix around again and so it was a bit of a learning experience again for the race," said Hartley.
"I'm sure we will get some good results next time out as the car's been quick at times during the season."
Point's table
* Robert Wickens 198
* Jean Eric Vergne 164
* Daniel Ricciardo 125
* Albert Costa 120
* Kevin Korjus 118
* Alexander Rossi 116
* Brendon Hartley 65
* Nelson Panciatici 48
