Ferrari and McLaren are embroiled in this year's 'fight for third', with just three and a half points separating the two teams and just five races remaining.
In Russia, Charles Leclerc was the first Scuderia driver to use the new system followed by Carlos Sainz in the subsequent race in Turkey.
With Ferrari clearly having the edge at the last race in the United States when both drivers ran the new PU, Ricciardo said: "I don't want to be too pessimistic but they have made that little step that has put them a nose in front
"Let's say, on probably the majority of circuits we'll go, we might find the odd circuit where we've got a bit more but I would say, generalising it, they are a little step in front now.
"A bit like in Austin we're going to have to work pretty hard to keep them at bay in the constructors'."
Asked as to how much he was enjoying the battle with Ferrari this season given the history between the teams, Ricciardo replied: "It's fun. It's cool to be fighting with them.
"Honestly, it doesn't change too much if it's them or someone else but everyone knows Ferrari.
"It keeps more people interested when McLaren and Ferrari are two of the biggest teams in the sport. It's cool to have a historic battle."