Brawn's team pushed the Red Bulls close at the start of the season but have seen the championship move away from them with Sebastian Vettel's recent dominance.
The Mercedes boss, however, said is confident that his team can continue to close in on Red Bull.
"It is frustrating and clearly they have got very, very good performance at the moment," Autosport quotes Brawn as saying. "But that motivates us to do better.
"It looks to us as though they made a step change around Spa time, and probably in our car some of the changes we have made to improve the race performance have slowed down the single lap performance. That is the compromise we are playing, but they have been a very strong reference for everyone the past few years.
"We are getting closer. I was looking at some data from last year, and it is difficult to compare completely because last year we had DRS on the whole track, but we are 1.5 seconds faster in sector one [in Japan] than we were last year, with basically the same tyres."