Dutchman Verstappen leads seven-time champion Hamilton by eight points heading into the final two races of the 2021 season.
But the Brit has reduced the gap in recent weeks after back-to-back victories in Brazil and Qatar.
And with the Saudi Arabian GP set to benefit cars built for faster circuits – such as Mercedes – Horner feels it is a track Hamilton will be confident of winning.
Speaking to RaceFans, he said: “I think it’s going to be tight, The next track arguably should favour Mercedes. Abu Dhabi, with the modifications made there, who knows.
“But it’s been incredibly tight so we go into those races eight points in the lead in the drivers championship. We’ve reduced the championship lead in the constructors to five points.
“So both are fully in play and that’s fantastic as we’re now at the climax of this of this world championship.”
Along with the drivers’ championship, the constructors classification also remains in the balance heading to Saudi Arabia and then Abu Dhabi.
Mercedes lead by five points but that could all change if Verstappen and Sergio Perez impress across the weekend.
No team has managed to wrestle the constructors title away from Mercedes since their era of dominance began back in 2014.