She's smart, but if a big(ger) hitter hits a firm shot away from the middle of the court, she is instantly on defense and cannot do anything. Li Na did it all the time last year, and did it a lot even in a losing effort in Sydney. Radwanska's skills come into play when the rallies are long and medium-paced, when she can try to exploit angles and width slowly. Her serve is not good enough either to guarantee holding serve. And because she does not return with authority, it is not a given she'll break frequently either, unlike Azarenka.
Basically, she has to serve incredibly well and make no unforced errors while redirecting hard balls while on top of the baseline, to be able to beat a big hitter.