George Russell admitted his pace in the United States Grand Prix was "rubbish", with the Mercedes driver at a loss to why he trailed race-winning team-mate Kimi Antonelli by such a large margin.
Russell finished 54 seconds behind the sister Mercedes in the 56-lap race in an eighth place that only just kept him in mathematical championship contention heading to this week's race in Mexico, where Antonelli is set to clinch the crown.
Russell had held his grid position of sixth through the race's first stint but dropped to eighth after serving a five-second penalty at his sole pit stop for an incident with Oscar Piastri in the opening lap, when he was judged to have gained an advantage over the McLaren by running off track and retaining position.
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Russell subsequently spent the final 26 laps on the similarly one-stopping Piastri's tail but was unable to find a way back past.
Russell told Sky Sports F1: "The pace was rubbish. That's the long and short of it.
"The five-second [penalty] didn't help things and probably cost me two or three positions but the pace wasn't good enough when it needed to be."
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Russell made no excuse for his lack of competitiveness and said he had no immediate answer for why his Austin weekend was such a struggle.
"Just not been quick enough," he added.
"I need to try and understand why that is. It's strange when you're so far off.
"Hopefully there are some answers somewhere but at the moment I don't have them."
What went wrong for Russell?
Antonelli's win was his ninth of the year and represented what Toto Wolff, Mercedes' team principal, believes was "the best race that he's delivered so far".
It was the first win for Mercedes upgraded car, which has taken time for the world championship leaders to get the best out of since the aerodynamic revisions debuted at the Bahrain Grand Prix in Malaysia at the start of October.
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The team have said, however, that the changes have narrowed the car's optimum set-up window.
On Russell's weekend, Wolff said: "George just struggled so much. And I believe this is because the operational window of the car is so difficult.
"In Q2 he was there and thereabouts. Q3, it didn't work. He said, 'that goes on me'.
"And today in the race, I don't know, we got stuck behind Piastri, which looks a bit unfair why he didn't get past. It's the same engine, it's the same electric charging and deployment. But I'm going to see him now."
'He's not 54 seconds slower over a race distance'
Analysis from Sky Sports F1's Jenson Button:
"George has had a lot of tough races but also some great races and that's the thing that's really difficult to understand - how he's able to get the maximum out of that car on some tracks but not at others.
"Austin was difficult as he started so far down. He was in the pack and when you're in the pack, sometimes it's difficult to move forward. You're in the dirty air. You try to do something different with strategy but then you get a five-second penalty which puts you back further.
"I don't believe his pace is that far off. He's not 54 seconds slower over a race distance."
Formula 1 next heads south for the second leg of The Americas triple header with the Mexico City Grand Prix this weekend, live on Sky Sports F1. Stream Sky Sports with NOW - no contract, cancel anytime