Valtteri Bottas admitted he struggled on the brakes compared to Mercedes Formula 1 teammate Lewis Hamilton in US Grand Prix qualifying, after lapping nearly half a second slower in Q3.
After a difficult day of practice on Friday, where he ended up more than six tenths down running a different set-up to Hamilton, Bottas closed to within 0.214s of his teammate in final practice at Austin on Saturday.
But Bottas remained behind in qualifying and fell further away in the final stages, ending up 0.460s down on Hamilton and knocked off the front row of the grid by a late improvement from Sebastian Vettel's Ferrari.
Bottas lost most of the time in the final sector of the Austin circuit and said he'd been struggling under braking, particularly in Turn 13 and 15 – the slow-speed hairpins.
"Definitely the last sector in qualifying, I did struggle more," Bottas admitted. "It felt more tricky, for some reason, in some of the practice sessions.
"There have been things I have been struggling with - brake modulation and locking, transferring weight. I have struggled to get it together and many times 13 and 15 I lost a bit of time - those are the longest corners.
"Sometimes I got it right, but even so I couldn't match Lewis in terms of cornering speeds, etc. It was looking good for us, and quite close with Lewis, so disappointed."
Bottas has struggled to find consistent form in recent races, and hasn't beaten Hamilton in qualifying since July's Hungarian GP.
"It was not easy session, every lap a bit different," Bottas added. "There was no big mistake, there was just this big gap.
"The first run felt okay. Lewis managed to gain more from Q2 to Q3, [but] I couldn't find any track improvements [and] I couldn't improve in the second run.
"It was quite tricky session with the wind, we have been making big set-up changes in the weekend, have been making car quicker, but tricky to drive - for me at least.
"The laps felt good, but just couldn't go quicker."