Lando Norris said McLaren were “miles off” the pace after the first day’s practice at the Azerbaijan Grand Prix.
The world champion has won two of the past four races, and would likely also have been victorious in the last race in Madrid had it not been for the unfortunate timing of a virtual safety car.
McLaren were hoping for a step forward in Baku after introducing a significant upgrade package to their car.
But Norris was sixth fastest, 1.484 seconds off the pace set by Mercedes driver George Russell, after the first day’s running.
“We’re up there,” Norris said. “Relatively, we’re in the position we expect to be in, but I’m 1.5secs off the pace, so pretty pants.
“We’ll see what work we can do tonight but it’s a little knife in the heart when you see how far off you are.”
McLaren are one of several teams to bring a major aerodynamic development to Baku, theirs focusing on revised bodywork shaping around the sidepod air inlets, engine cover, floor and diffuser.
Chief technical officer Rob Marshall said: “We’ve introduced a significant upgrade this weekend and the initial data suggests the car is behaving as our simulations predicted, which is a positive starting point. However, it’s clear we haven’t yet unlocked its full potential.”
Russell headed both practice sessions, with the F1 weekend running unusually from Thursday to Saturday because it is Azerbaijan’s national remembrance day on Sunday.
The Briton led team-mate Kimi Antonelli by 0.552 seconds in the second session. Red Bull’s Max Verstappen was Russell’s closest rival in the first session, when Antonelli suffered a hydraulic failure, and third fastest in the second session.
Antonelli’s reliability problems also meant he missed the first half of the second session, and overall he had half the amount of running of Russell.
The Italian, who leads the championship by a massive 81 points from Russell, said: “We tried to do the max with what we had. Lost quite a few laps in FP1 and then missing half off FP2 is never ideal, especially in a street circuit.
“But overall I feel it was a good recovery. I feel quite good in the car. A few things to work on but looking forward to tomorrow.”
McLaren’s rivals rejected the idea that they were struggling badly.
Red Bull technical director Pierre Wache said: “Mercedes are quicker. McLaren looks a little bit quicker or similar to us.”
Ferrari team principal Frederic Vasseur said: “The McLaren was very quick but they never finished their lap.”
And Mercedes deputy team principal Bradley Lord said he did not “think the gaps are representative”.
