All bar one of Sainz's four seasons in F1 have been with Renault power, and he drove for the French manufacturer's own team at the end of 2017 and all of 2018.
After an encouraging qualifying performance in the season-opening Australian Grand Prix, Sainz said he could "feel a change" in Renault's competitiveness.
"I feel a step in the right direction," said Sainz.
"I would praise Renault in that sense because it finally seems like they've done the step that we were needing two years ago."
"I trust Renault, they've done steps, so hopefully we can keep pushing," he said after the grand prix.