Lewis Hamilton said his Mercedes team “desperately” need car upgrades after a difficult Friday practice at the Miami Grand Prix.
The seven-time champion was 0.928 seconds slower than Max Verstappen’s Red Bull, which set an imposing pace.
Hamilton said: “It’s a kick in the guts and it’s a little bit difficult to take sometimes.”
Mercedes are due to introduce a major car development at the next race, the Emilia-Romagna Grand Prix on 19-21 May.
“We desperately need those upgrades,” Hamilton said. “We have to keep our head down for one more race and hopefully we will start a new path next race.”
Friday started positively for Mercedes, with George Russell leading Hamilton in a one-two in first practice at the Hard Rock Stadium on a hot and sunny Florida day.
But team principal Toto Wolff gave a hint of what might be to come when he said after the first session: “The car has some goodness, it is just very tricky to unlock it. When the drivers have confidence, they can go quick.
“It is on a razor’s edge. This morning we seemed to be in a sweet spot. I don’t think we can extrapolate from this.”
And Wolff’s comments turned prophetic in a second session in which Hamilton was seventh fastest and Russell down in 15th, 1.286secs off world champion Verstappen’s pace.
Hamilton said: “Same as every weekend – we are a second down. [Miami] is a great place to be but we are not particularly quick and it’s a struggle out there. P1 looked quite good but in P2 the true pace came out.
“I am going to stay hopeful that we can get the car in a better place. We had the extreme bouncing last year but it feels like we are racing the same car.”
Russell added: “Similar thing to last year, when we were quickest on Friday and then knocked out in Q2 on Saturday.
“The car just changed even from the medium [tyre] run in P2. I felt strong, was on a good lap which would have put me in the top four and then put the soft tyres on and the car just wasn’t working for me.
“I think we understand a little bit why that is and have time to improve things overnight. Fine margins.
“If we get things right, there is no reason we can’t be ahead of Ferrari and Aston Martin.”
At the front, Verstappen set an imposing pace as he headed the Ferraris of Carlos Sainz and Charles Leclerc.
The Dutchman was 0.385 seconds quicker than Sainz, with Leclerc just 0.083secs further adrift.
Leclerc crashed late in the session during the race-simulation runs, causing a red-flag period and limiting teams’ abilities to assess their cars on heavy fuel loads.
Red Bull’s Sergio Perez was fourth, 0.489secs off his team-mate.
The Mexican, who is six points adrift of Verstappen in the championship and said on Thursday that he would have to be “perfect” to have a chance of challenging for the title this year, said he “hadn’t had the greatest of Fridays”.
“We have been trying a few things with the mechanical balance and we learned quite a bit,” Perez said.
“My lap was also pretty bad. I locked up on the final corner so there is quite a bit to come. I am not driving really well today. So if I can improve my driving and get myself a bit more comfortable, it should be fine.”
Verstappen, by contrast, said he had had “a positive day”.
“Initially it was getting used to the track a bit with the new Tarmac,” he said. “It was ramping up a lot through the day but I always felt good in the car and we had a good balance in it so I felt happy.”
Aston Martin’s Fernando Alonso was fifth, ahead of McLaren’s Lando Norris and Hamilton.
Norris said he had had “one of our best Fridays so far” but he added that he felt he had done a good lap that had flattered his car’s overall competitiveness.
Aston Martin’s Lance Stroll, Alpine’s Esteban Ocon and Williams driver Alex Albon completed the top 10.
Leclerc crashed with about eight minutes to go. The rear twitched through the long right-hander at Turn Seven; Leclerc caught the moment but could not stop the car nosing into the barriers.
The front-right corner and front wing were damaged but the new floor introduced as an upgrade by Ferrari this weekend appeared unscathed.
Drivers had only just begun their race runs at the time, their attempts to gather data already frustrated by heavy traffic on the temporary street circuit around the Miami Dolphins’ Hard Rock Stadium.
The session did resume after the accident, but the lost time meant all the drivers will go into Saturday with very limited information as to their race pace.
And it meant the usual comparisons between the cars on full tanks – which can provide a more accurate analysis of true pace than the one-off headline lap times – were not applicable.
In the first session, two drivers damaged their cars, Nico Hulkenberg crashing his Haas at Turn Three and Alpha Tauri’s Nyck de Vries spinning and brushing the wall half way around the lap.