The Dutchman jumped to the top of the leaderboard after lunch, taking the advantage on the supersoft tyre compound after Sainz, the Spanish driver who finished seventh in his home Grand Prix just two days ago, had gone quicker than Ferrari’s Sebastian Vettel.
And the top three remained that way, while Mercedes’ Lewis Hamilton had to settle for P6, as all 10 teams developed their set-ups in what is the first in-season test of 2018. Indeed, eight of them clocked up over a century of laps at this familiar track, and it was Hamilton who did the most with 151 in his Silver Arrow.
Vettel ran 136 laps in the SF71H, with the German and his Italian team looking to understand their car more after a disappointing race in Spain saw them struggle for pace behind the dominant Mercedes.
Romain Grosjean set the pace in the morning segment, and that time was enough to put the Haas driver fourth overall. It was a welcome boost for the Frenchman after his Turn-3 spin wiped him, Nico Hulkenberg and Pierre Gasly out of Sunday's race on the opening lap.
Force India's Nicholas Latifi enjoyed an impressive first run in the VJM11, finishing ahead of Hamilton – winner of the Spanish GP – and seventh-placed Stoffel Vandoorne in the McLaren, who managed 85 laps on day one.
Antonio Giovinazzi guided Sauber to eighth, and will return to the circuit on Wednesday to take over from Vettel in the Ferrari. British talent Oliver Rowland’s debut for Williams was respectable, with 121 laps en route to P9.
Pirelli's 2019 tyre development programme is also taking place alongside this test schedule, with McLaren and Force India each providing a car. For the former Lando Norris shared day-one duties with fellow Brit Oliver Turvey, while at the latter Mercedes junior George Russell was at the wheel.
Norris set the tenth-quickest lap of the day, but it was a frustrating session for Toro Rosso and their driver Sean Gelael. A chassis-related issue forced them to change their power unit as a precautionary measure, and ultimately restricted them to just 50 laps.
The Spain test continues on Wednesday. For line-up details, click here - and for minute-by-minute timesheet updates from Barcelona download the Official F1® App and go to Results> Testing. For more information, click here.
* Conducting Pirelli 2019 tyre development programme