Austin - Mercedes team chief Toto Wolff warned on Saturday that he fears Ferrari's resurgent speed could create an upset despite Lewis Hamilton claiming pole position for Sunday's potential title showdown United States Grand Prix.
A relieved but still concerned Wolff said he half-expects the drivers' championship may not be decided in America this weekend.
"I am very happy about taking pole, but it's so close between the top three," he said. "My thinking is already about tomorrow and that we are lacking some straight-line speed so I expect Ferrari will be attacking us.
"I would say that you can't write Ferrari off and it has been close through the whole season so it will be close all the way to the end of the season.
"It's about getting the best possible race result and if it doesn't happen tomorrow, for us, then we will try again in Mexico and try and get one step closer."
Mercedes' Hamilton took pole by a narrow margin ahead of his only remaining title rival Sebastian Vettel of Ferrari. Despite qualifying second-fastest, Vettel will start from fifth on the grid after collecting a three-place penalty in practice on Friday.
That means four-time champion Vettel's Ferrari team-mate Kimi Raikkonen will start alongside Hamilton on the front row.
Hamilton will clinch his fifth drivers' title if he can outscore Vettel by eight points.
The 33-year-old Briton said: "That was very important, it was so close.
"These guys (Ferrari)!.. I didn't know how close it would be. I knew it was edgy between us and would require some solid laps.
"The first one was decent, but not enough and the second one was better.
"There's been some races when I haven't been able to do better so I'm happy I could today.
"The track started out green and slippery and it's been getting faster and faster in every session here this weekend. There's also a head wind going into the Esses... But it's such a big crowd here - and there's so many fans.
"We'll see what happens, but the Ferraris are quick. I hope for better weather for everyone watching - and I hope it's an exciting race."