Who can beat Serena at Australian Open?

Question

Now that Kim Clijsters has lost, Justine Henin surely has to present the biggest threat to Serena Williams’ attempt to defend her Australian Open title. The pair wouldn’t meet until the final though, and Henin has a string of potentially tough matches to get through before then. Serena also faces a potential semi-final against sister Venus, but Serena has taken the upper hand in their head-to-head matches in recent times.