Formula One leader Oscar Piastri said his vanishing speed was "a bit of a mystery" after struggling to seventh on the grid in Mexico City Grand Prix qualifying while teammate Lando Norris took pole position.
Norris lapped the Autodromo Hermanos Rodriguez on Saturday with a time 0.6 seconds quicker than the Australian. Ferrari's Charles Leclerc and Lewis Hamilton were second and third.
Piastri qualified eighth but moves up a place thanks to Williams' Carlos Sainz carrying a five-place grid penalty from Austin.
"I'm a bit frustrated with how the session's gone," Piastri told reporters. "There's a lot of things I could worry about, but ultimately, being that far off when you feel like you've done a reasonable job is a difficult place to be."
The gap between the McLaren teammates has been stubbornly consistent all weekend, leaving Piastri searching for answers. "Clearly one of the cars was quick (today), so we need to try and work out why I'm not," he said.
What was particularly frustrating for the 24-year-old, who is 14 points ahead of Norris and 40 clear of Red Bull's Max Verstappen, who qualified fifth, was that he felt he had not fundamentally changed his approach.
"I've not changed really how I'm driving since the start of the season, and even a few races ago when things were going really well," he said. "It's difficult to pinpoint where the lap time's been lacking this weekend."
The issues echo problems from Austin, dashing hopes those were track-specific.
"In some ways, not too dissimilar," Piastri said. "Some of the things that were difficult in Austin are also proving difficult."
Despite the frustration, he remained optimistic about Sunday's race prospects.
"I'll try my best. I think if I can unlock the pace of the car, then we can have some fun. We're just going to try and unlock it."