Red Bull driver Max Verstappen said he was not surprised to see Ferrari struggle so much during Sunday's United States Grand Prix amid recent scrutiny of its car and a recent technical directive issued by the FIA.

Ferrari enjoyed a surge in form following the summer break, taking three straight wins and six consecutive pole positions as both Charles Leclerc and Sebastian Vettel took the fight to Mercedes and Verstappen's Red Bull, having struggled through much of the European season.

Much of Ferrari's strength was put down to its engine and top speed, leading to questions from some corners over how it could have been achieved given the maturity of the current engine regulations.

Red Bull went to the FIA with a question regarding a potential fuel system solution, only to be told ahead of the United States Grand Prix that it would not be legal to run, resulting in a technical directive being issued to all teams.

Leclerc finished a distant fourth in Austin on Sunday, 50 seconds behind race winner Valtteri Bottas, while Vettel retired early on after slipping back from second to seventh on the opening lap.

Asked if it was strange Ferrari could not fight at the start, Verstappen replied: "Oh that’s not strange," before adding "well why do you think? You can fill it in yourself" when invited to explain his reply.

Informed that Vettel had reported a lack of grip on the opening lap, Verstappen said: "Yeah yeah, I would say the same."

P3 in Austin drew Verstappen to within 14 points of third-placed Leclerc in the drivers' championship, keeping the Red Bull driver's hopes of a podium finish in the final standings alive.

Verstappen was asked if his view on the battle for P3 had changed, with one reporter suggesting it may have swung in his favour "now that Ferrari may not have the same-" - at which point the Dutchman butted in: "Power you could say? Or grip, I don’t know what it is!

"We always try. For me of course, only first counts, but it’s still better to finish third than fifth of course."

Ferrari has long defended the legality of its power unit despite the speculation, and invited rivals to lodge a protest at the last race in Mexico, but no formal action was taken.

Leclerc was also forced to switch back to an older-spec Ferrari power unit in Austin following an issue in final practice on Saturday.

Additional reporting by Julianne Cerasoli.