Red Bull chief technical officer Adrian Newey has conceded he was concerned by Ferrari's potential ahead of pre-season testing prior to the start of the past campaign.
Max Verstappen and Red Bull ended up dominating F1 this term, with the Dutchman scoring a record-breaking 15 race wins en route to his second title.
But given the intense championship battle with Mercedes throughout 2021, Red Bull was limited in its ability to develop the RB18 for the new era of technical regulations - a problem that had not hindered Ferrari with its F1-75.
Reflecting on the situation now, Newey said: “In pre-season testing, we were a little bit worried before we got there.
"We’d already done a little bit of research and knew roughly what we needed to do to improve it [RB18], so when we put the race one package on in Bahrain just before the race, that catapulted us from definitely behind Ferrari to broadly level.
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“The car definitely had some weaknesses in the first half of the season and still had some [at the end], but we reduced those weaknesses, and certainly by the second half, we had a fully competitive package.”
The car went on to score 17 race wins and eight pole positions, and Newey added: “Statistically, obviously RB18 has been our best car.
"It’s a car I think we can be very proud of in as much as we had a tight championship battle through ’21, obviously, so we kept developing the car well into the season.
“And of course, when you have limited resources, if you’re putting research and development into that car then that research and development are not going into this brand new car for the new regulations which we knew were coming.
“It’s a difficult balancing act, but we focused on trying to get the fundamentals right, trying to get the package in a way that, even if it didn’t start out as the quickest car, we could develop through the season."