The Isreali driver drove the car for the first time during the afternoon at the Yas Marina Circuit, taking over from George Russell who drove the afternoon session.
Nissany racked up 41 laps during the session - but states that he spent little time becoming content with the car.
"Firstly, I feel great," he said after the test. "You know that feeling that you like your job and you know what you are doing. It took me four laps to get comfortable in the car, and from there on it was just building up my pace and confidence.
"We were improving and making our best laps until the end, and we made a few changes according to my wishes. I now feel confident in the car, with a lot of potential in our pocket so that tomorrow morning will be exactly what we need for the last improvements in 2019."
Nissany has previous experience driving F1 cars, having tested the 2012 Sauber C31 in 2014.
Nissany will be back in the Williams car on Wednesday morning before George Russell takes it back out for its final on-track display of 2019.
Russell described the opening day of the test as "productive" as Williams ran "five sets of 2020 tyres and gathered some useful information", which will help the team next year.
Bahrain International Circuit - Winter testing