Red Bull Team Principal Christian Horner says the team will need to be at "the top of our game" to beat its rivals at Singapore.
The team has struggled to show the same performance out of the box as it has done in previous races, with Sergio Perez and Max Verstappen ending practice seventh and eighth fastest.
Both drivers complained about the lack of rear grip during their runs on the Soft tyre, but in the long runs, managed to get closer to pacesetter Ferrari.
Ferrari's times present a significant threat to Red Bull's win streak, as it looks to become the first team in the history of Formula 1 to win every race in a season.
"This track is going to be one of the toughest of the season for us," Horner told Sky Sports F1.
"The characteristics don't play naturally to the strengths of the car and I think that we're going to have to be right at the top of our game to keep this winning momentum going.
"You can see Ferrari look strong, McLaren and Mercedes are there as well, so it could be very, very tight this weekend."
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Horner: Singapore more of a challenge for us
Horner explained that the Marina Bay Street Circuit is one of the outliers on the calendar, which does not play to the characteristics of the dominant RB19.
"Predominantly high speed has been been strong for us, combined corners as well or combined entries," said Horner.
"Whereas 90-degree corners, we saw it in in Monaco and Azerbaijan even earlier in the year, they've been more of a challenge for us."