Serena actually hasn't been playing that well in Miami, prior to last night. Of course, part of that, IMO, could be the matchup with Radwanska. She knows Radwanska doesn't possess anything to really trouble her, so she's probably more relaxed.
Li Na, on the other hand, can hold her own with Serena on the groundstrokes. Their matchup was a pretty good match.
Serena, as always, struggles to stay motivated and focused on tennis. She got fired up after the French last year and won pretty much everything from that point on. Now, she seems to be back to her standard self...which is great, but not "Super Serena".