I dont think Clijsters is an easy opponent. She was a bonafide top 4-5 player in that very deep earlier 2000s field of Venus, Serena, Henin, Clijsters, Davenport, Capriati, and others all at their peak together. In her prime she managed only 1 slam but many including you picked her as the best player to win only 1 slam just ahead of Sabatini. She won 37 career titles despite retiring at 23, and only won 1 slam due to a combination of some bad luck, some ill timed injuries (she could have won 2 or 3 more slams in the 2004 year she got injured alone which Serena, Henin, and Venus were all surprisingly quiet in, horrible timing), the killer competition back then, and her whole psyche issue with Henin. Now she is back looking to be almost playing as well as before. Many consider her a danger to Serena in that semi and are discussing her a possible winner now, so if Serena does beat her it certainly should not be dismissed as "weak" competition the way I would concede it could be for the lesser Russians and Serbs. Clijsters is a legit opponent, not quite as much as Venus and Henin, but one nonetheless.
pmerk34 is right that Serena is so good and so powerful she is a threat to win slams in any shape, even in a stronger era. In nearly any shape she has that serve, and boy that serve isnt easy for any women to return. She has the power, very good groundstrokes, decent volleys, and incrdible strength of mind. By my last comment who has saved as many match points on the way to slam titles as her. Just incredible. Even if some of those were vs mental midgets and chokers, which yes some of them were, she still had to hit a number of shots to save atleast one match point and play a quality point on her tournament would have been over. Just not an easy out, regardless if at her best or not. The one thing I would concede is in her current shape, which still isnt 100% for her, she wouldnt be able to win the French Open on her worst surface in other eras, but even in this era she isnt winning the French Open anymore it seems so who cares. She also is in much better shape than many people seem to think she is. If you look at pictures of her close up on a tennis court she is quite fit at the moment. Not as fit as 2002-2003 but still definitely in good shape. People seem to forget she is a big women by nature. That is just her body type.