Cape Town - Valtteri Bottas admits Red Bull have made notable gains in recent weeks, saying they are "even stronger" than Mercedes expected.
Max Verstappen won the penultimate race of the season from pole to flag and Red Bull only missed out on a second podium spot after Alex Albon was taken out by Lewis Hamilton during the closing stages.
Worryingly for Bottas and company is the fact that Red Bull are now "better than Mercedes" on the straights.
"We knew they would be strong, but I think they were even stronger than we expected," the Finn told Racefans.net. "Especially in a straight line.
"They have a good car in the corners now but in a straight line, they are better than us now. It used to be pretty similar to us."
Mercedes had a big advantage over the rest of the field at the start of the year, but Ferrari eventually narrowed the gap and now Red Bull have also caught up.
Asked if this will make for a more competitive start to the 2020 season, Bottas replied: "If you look at the beginning of the season to the end of the season, I think Red Bull has made some solid progress. So if you would draw that graph till next year, yes.
"But obviously it's a completely new season with a new car. I'm sure every team has made progress, including us, with their new car that is going to be there in testing.
"So it's impossible to predict, really. Everyone starts from zero, even though there's not many regulation changes it's still new cars."