McLaren driver Carlos Sainz Jr believes "teams like Ferrari and Red Bull" are capable of lapping "much faster" than his own benchmark in Formula 1 pre-season testing at Barcelona.
Sainz's team-mate Lando Norris set the pace on the first day of the second week of testing, before Sainz took over in the MCL34 and posted the fastest lap of the F1 pre-season so far - a 1m17.144s.
That lap, set on the C4 compound, was a quarter of a second faster than Nico Hulkenberg's opening-week benchmark and is within a second of Mercedes driver Lewis Hamilton's Spanish Grand Prix pole time from last year.
Asked how close he had been to qualifying trim on that lap, Sainz said: "I think it's not for me to comment on that, but if you are clever enough to see our kind of running, you would guess that it was a good try."
Sainz described the lap time as "fast, I guess", but conceded McLaren was "still not where we want to be".
"There's still big areas of improvement," he added.
"The important stuff to come out of this seven or eight days [is that] every time I've been on track the car has been progressing.
"I think our car balance has gotten better over the last few days, and that is allowing us to extract a bit more from it.
"But still there is a long season ahead where we need to keep working, we're still not where we want to be.
"This is definitely the right answer, and we need to keep working hard because a [1m]17.1s around Barcelona is fast I guess, but I expect teams like Ferrari and Red Bull can go much lower and much faster than that.
"So I'm very cautious and very calm."
Sainz said "a lot of stuff" has changed on the MCL34 since its track debut.
"Pretty much everything has kind of been improved," he said.
"There's still underlying balance issues that we are working on and that we cannot get rid of until we bring upgrades to the car.
"So that's what Formula 1 is about: decide on what you need to improve, and in the following races try and bring upgrades to make it a bit better."
McLaren has already managed more laps than it had accrued in pre-season testing in 2018, but is only seventh in the mileage rankings this year.
Sainz said the team's Barcelona test programme has not been without issues, but stressed these were to be expected.
"Every day you'd like to do a couple more runs, but that is what testing is for - because if you don't find the reliability issues in testing like we are finding now, it means that you're not covering enough work," he said.
"We are finding issues, but we are solving them and we are getting back out on track like today and we did 130 laps with a couple of issues.
"So that's good news and good progress."