Sainz posted an impressive time of 1:19.837s on Friday. Despite some early glitches in the first week of testing, he completed 224 laps (1043km) in the second.
"What a great feeling to end pre-season testing with such a good day," he said afterwards. "I’m very happy with the lap time and with the number of laps completed.
"Managing to cover a full run plan was exactly what we needed before going to Melbourne," he continued. "We can now be a lot more positive about the season that’s about to start.
"It’s been a tough couple of weeks for the whole team, with problems here and there, but today we finally managed to put a very good day together
and we’re definitely ready to race."
Sainz predicted a tough battle in the mid-field of the pack when F1 finally takes to the track in Australia in two weeks time.
"It is going to be a very very tight fight for P5 in the championship, or to be the fifth-fastest car in Melbourne," he said. "Yo get into Q3 will be extremely difficult.
"Williams is flying," he pointed out. However, he was circumspect as to whether Williams would prove to be the team for Toro Rosso to target.
"I don't know, it's probably a bit too early to know. But they are definitely very strong and they will be very difficult to beat.