With the Beijing title, Sharapova not only moves into the #2 ranking, but she also won her first hardcourt title in over a year and a half (Indian Wells 2013).  If Serena doesn't make it to the WTA Championships, 1500 points will drop off, leaving her less than 500 points ahead of Sharapova, meaning that the #1 spot could swap in the next few weeks.  If Kvitova and Halep play more events, they could conceivably overtake Serena as well (I cannot figure out from their terrible websites whether they are playing more or not).