“I think every single team, at some point of the tests, they've done a very strong laptime, so it's very difficult to know who is in front of the midfield or behind,” he said.
“I think the Racing Point and even the AlphaTauri, I would put them as probably the most dangerous guys to beat.
“And since then they haven't improved much because I don't think they want to - but that 17.3 on the first day, I think it impressed all of us and they're going to be quick, very quick.”
McLaren finished last season at the head of the midfield in terms of points scored, but Sainz expects the battle behind the leading three teams to be even more tightly contested this year.
“We've added some good downforce to the car and we're happier than we were 12 months ago. The car feels much better, especially in sector one and two [at Barcelona] we’re very well balanced.
"The last sector today looked at bit better but still our weakness and something that we need to be working on as a team.
“I think there's still differences in how teams are running in fuel loads or in engine modes [in testing] and we need to take that into account.
"We cannot take too many conclusions, we need to keep analysing ourselves, and I think we we're doing a pretty good job on dissecting our performance levels in different sectors and trying things to make it better.
“And the differences you're trying to spot, you see 10 kilos is three or four tenths [of a second]. So it's impossible to take a conclusion because with three tenths in the midfield you go from being P7 on the grid to being the P17.
“So who has the last three tenths in the pocket? Or who has used the last three tenths of the car? No one knows.”