However, Haas and even Sainz's former team Toro Rosso have looked faster.
It has moved Dr Helmut Marko to note that some teams targeted Red Bull before the season, but are now struggling even to match the junior team Toro Rosso.
Sainz, who is still in the Red Bull driver programme, told AS newspaper: "Helmut is someone who says things very directly and very clearly and when he is right, he is right.
"Right now we do need to improve -- I said it in Bahrain," the Spaniard said ahead of the Chinese grand prix.
"I was not comfortable with the car, we lacked grip on the rear in the slow corners, at the front in the fast corners, and with that balance it's very difficult to lead the middle of the grid."
But Sainz said he is sure the bigger midfield teams like Renault and McLaren should be able to out-develop smaller outfits that currently look better.
"I think, and it's at least the objective that we have, that there are teams like Renault and McLaren that should take bigger steps than the others.
"We know that some like Toro Rosso took a very big step, but we hope over the whole year to make an even bigger step. That's the goal of a factory team like Renault with aspirations to become world champion in a couple of years," Sainz added.