The qualifying sessions saw a number of different race strategies adopted, with Mercedes's Lewis Hamilton and Nico Rosberg opting for the soft compound tyres. Hamilton finished in pole position for the first time since he joined Mercedes.
"I think it's going to be a very complex race tomorrow," Brawn told Sky Sports News.
"We don't really know what we've got. The work we've done on Friday and today obviously helps a lot to try and understand what we've got to do but it'll open up in one direction or another.
"I think it's going to be a really great race tomorrow. It's one which we need to be completely on top of - we know what we need to do - and even on the prime tyre you can't go flat-out for 20 laps - you've got to manage the situation as well."
Mercedes are currently in fourth place in the 2013 constructors' championship.