The Spanish driver spent the afternoon conducting runs on various compounds, focusing on understanding the tyres.
"I think today was a much better day compared to last week and we've definitely made a step forward in terms of reliability," Sainz said.
"Our test programme this week is a very tight one as we still have a lot of catch-up to do, so it's therefore still too early to make any conclusions.
"Even though we lost a bit of valuable track time, I'm happy with the fact that I was able to do so many laps, because that's what testing is all about!"
Jody Egginton, Toro Rosso's head of vehicle performance, was satisfied with the team's day of work although a few niggling issues still crept in.
"A day of mixed fortunes really, with some very good steps forward made with the set-up but, at the same time, we have been dealing with some small but frustrating background issues which have broken up our running and cost some track time," explained Egginton.
"Looking forward to tonight's activity, the mechanics, who have been putting in a massive effort in the last days, have a very busy night ahead of them preparing the STR12 for tomorrow's running.
"At the same time, the engineers will be working through the data from today in order to optimise the starting set-up and test plan for tomorrow when Daniil gets back into the car for his final day of pre-season testing."