Hardenne has never beaten the Williams sisters on anything but clay. She has a 5-3 record against Serena, and a 7-1 record against Venus, she's not really in their league on anything but clay, let's be honest.
Its all well and good saying "Yeah all they do is hit the ball hard", but they win the clutch points, they don't choke like a Clijsters in a semi-final when the match is there to be won. When they are match point down or two breaks down in the 3rd set, you can never guarantee that you'll beat them, particularly Serna. See Clijsters vs Serena SF 2003 Aus Open, Sharapova vs. Serena SF 2005 Aus Open, and countless matches this tournament Serena has been in. That's totally different than Hardenne dominating against an Ivanovic or Kuznetsova or *insert female power player here*.
Mental toughness, I don't see it in Hardenne, Mauresmo and definitely not Clijsters. That being said I reckon Clijsters could take the Australian Open out this year, with many gun players out already and Serena Williams still not completely at her best.