The only player that can challenge Serena on hardcourts (other than Venus) is...
Of course assuming Serena is healthy and on.
Well, let's analyze the top 10 + Hantuchova.
Jennifer Capriati: Her problem is that her game is mostly based on speed and power, and in both she lacks behind Serena, she has lost the last 4 times the have played and even though some of those matches have been close, I don't see her beating Serena on hardcourts unless Serena is off.
Kim Clijsters: Again, although Kim does have finesse, her main weapons are speed and power, and right now, due her injury she is in a big slump.
Jelena Dokic: Similar case, she is too one-dimensional but way less powerful than Serena, no chances.
Justine Henin: As much as I would like Justine to beat Serena, I do think she can be a thread to her on other surface, definitely on clay, maybe on grass (not playing as she did last year), but hardcourts is Justine's worse surface and Serena's best.
Lindsay Davenport: again the common problem, power is her game and you can't overpower Serena, if she is serving extremely well and returning extremely well she has a chance, but her head-to-head against Serena is ugly, she seems to have a mental block against her.
Martina Hingis: Similar to Justine, she could have a chance on clay but on hardcourts, I don't see it happening. Martina could neutralize somehow Serena's serve a little with her anticipation but would get killed on her own serve.
Amelie Mauresmo: In theory she can manage power, but again, hardcourts isn't her best surface and she doesn't seem to be able to keep up long enough.
So, before Monica fans get upset for not including Monica, I left her for the last for obvious reasons.
Obviously, the Monica we have seen at Wimbledon and the FO can't challenge Serena, but if Monica gets fit.
She is probably the best returner of serve of the tour, along with Hingis, I am not talking about the ability to hit winners off the return, but to read the opponent's serve. Lindsay and Serena with hit more winners off the return, but also will fail more returns. Monica will nearly always put her racquet on the ball and return something difficult to deal with.
Monica isn't bothered by power in the slightest, she can manage it very well and hit a winner from behind the baseline, she actually feels more comfortable there.
If she gets to the ball in balance, she can hit a winner, it doesn't matter how fast the ball came, Monica's ability to strike the ball and her placement have allways been amazing, she totally controls where the ball is going and again, it's very difficult to overpower Monica.
The problem is that Monica is not moving well, I saw her at Wimbledon and I think her movement was terrible, specially when she had to move forward, she was actually having a lot of problems with short balls, but Serena would give her mostly deep balls with a lot of pace, something that Monica loves.
She needs two things, be confident and be fit, can she get fit on time for the US Open? I doubt it, maybe it's my way to say that I don't see anybody taking the US Open from Serena this year.