I think she better hope for a serious Serena let down. Serena would have to do a royal screw up for the rest of the year, and Li would have to remain consistent for the entire year for Serena to give up number 1. Though Serena has very little wiggle room aside from Wimbledon where she can seriously gain points, everything else she either made the final or won. She defends 2 of the last 3 majors. Though like I said, she would have to seriously mess up to lose all of those points. It used to be when clay season came I dreaded it for Serena, but she has proven to be quite consistent on this surface over the last two years. The only real area I am worried is when the French comes around perhaps the nerves she might have being defending champ.
Also, Li is a very capable player, but I do have to wonder what the injuries of both Sharapova and Azarenka have both had play into where we are at now. It may be that Li a very capable player would have moved past them anyway, or she would have hit a road block against them. It will be interesting to see once Sharapova gains some consistency and Azarenka returns what effect this will have on the women's game.