"The first impression was quite good – it felt like a step forward from last year, from the chassis, from the engine, from the gearbox side," said Nasr, who together with team mate Marcus Ericsson racked up over 2,000 kilometres in the Swiss team's 2015 car in last week's first test.
"It just felt like we've had a step forward on everything. As a back-to-back on the car from last week it just felt like we seemed to have a more stable rear when looking at medium to high speed corners.
"The engine has impressed me in the driveability being better - the whole response of the engine in all speed ranges being more progressive and consistent.
"It's still early days to be judging anything. We have to give time for the car to be developed, to get the set-up correct and we haven't done many mechanical changes yet.
"But a positive day, more than I expected - we did a lot of laps and got a lot of data."
Asked whether things might have been even more positive for Sauber heading towards the first race in Australia had the new car been introduced earlier, Nasr said: "If we do all the programme as we want over the next few days it will give us a good baseline for Melbourne, although it is true to say that if we had the car one week earlier it would have given us even more time to get all the areas even further developed.
"But it is what it is - we have to focus on what we've got now. As I said, there are some improvements on last year in terms of our limitations - on the braking side, with the rear of the car being more stable - so there are early signs of improvement, and I'm sure we'll see more progression over the next few days."
Nasr will hand the C35 over to Ericsson on Wednesday before completing his final day of running before Australia on Thursday.