The Italian, who had a few spins on Wednesday when the track was wet, completed more than 600km behind the wheel on Thursday at the Circuit de Catalunya and finished with a best time of one minute 21.900 seconds.
That compared with this year's fastest race lap of 1:22.762 set by Brazilian Rubens Barrichello and Jenson Button's pole position time of 1:20.527.
The 30-year-old had the advantage of a 2008 car on slick tyres and light on fuel but Ferrari insiders were impressed by the Italian's ability to learn and improve his times consistently.
Formula 1
Turkey drops off F1 calendar, Austria gets second race
14/05/2021 AT 16:14
"It shows he has the talent to progress," commented one.
Ferrari have tried to recruit Rossi in the past and team boss Stefano Domenicali said last week he would check on the Italian's availability if they had the chance to race a third F1 car in the future.
Rossi, who has talked more of a future in rallying after MotoGP, said he would "have given anything for such a lap time" in his sixth test in a Formula One car.
Brazilian Felipe Massa will take over the car on Friday as he continues his recovery from the serious head injuries he suffered in Hungary last year. Formula One testing in current equipment is banned until February 1.
"As the car and the track were available I thought it would be helpful to go onto the track for a day to continue gaining confidence with the Formula One," Massa said.
Formula 1
Hamilton wants new Mercedes contract before summer break
10/05/2021 AT 10:09
Spanish Grand Prix
Race Review: Mercedes show ingenuity to give Verstappen and Red Bull Hungarian nightmares
10/05/2021 AT 08:01