Oscar Piastri admits he is surprised at how Max Verstappen has charged into the world championship fight.
After Piastri's win in Zandvoort at the end of August, he led teammate Lando Norris by 34 points and Verstappen by 104.
But the Australian has not beaten Norris or Verstappen in any of the four races since and has seen his lead slashed to 14 clear of Norris and 40 ahead of the Dutchman as a genuine three-way battle has emerged.
Red Bull are expected to be strong again at a track where Verstappen has won five times and another successful weekend for the four-time world champion would continue his rapid rise into the title picture.
The 28-year-old, who declared his quest for a fifth successive championship to be over prior to the summer break, has beaten the McLaren duo at each of the last four races.
Asked if he believed after Zandvoort that his title fight was only with Norris, Piastri said: "I think probably yes but everyone did.
"The run of form that he [Verstappen] has had since Monza has been a bit of a surprise. There were flashes of that performance early in the season but also pretty big dips.
"He has come to the fight quicker than I expected."
Piastri endured a tough time in Austin last weekend, crashing in the sprint and also wiping out Norris before trudging home in fifth in Sunday's race.
But the 24-year-old says he is not thinking about the looming threat of his rivals.
"The gap has shrunk a bit in the last few races but the focus has always been on going as fast as I can," Piastri said. "I'd rather have the lead than being in any other spot. There is no benefit for me in worrying about that or focusing on that."
There are five rounds of the championship remaining, with 141 points still to fight for ahead of this weekend's battle in Mexico City.
Piastri struggled here last year, suffering a shock Q1 elimination before battling to eighth in the race.
The Autodromo Hermanos Rodriguez presents a unique challenge to the cars and drivers at over 2km above sea level, with the low air density at altitude making it more difficult to generate downforce.