"I felt like I was not playing any worse than over the last few years when I've started my claycourt season," said Clijsters.
"The big difference was the serve. She hit a lot of aces and good second serves and she kept me on my back foot a little bit.
"I was always at a bit of a disadvantage going into the rally already and didn't really get the chance to move her around too much. She hit really aggressively and kept me defending all the time.
"A couple of times I tried to step into the return a little bit and be more aggressive, but I felt she was stepping in even more and she went for her shots a lot. She was really going for it and hitting a lot of lines."
While Clijsters fell, fellow Belgian and number one seed Justine Henin swept aside struggling German Anna-Lena Groenefeld 6-1, 6-2.
Williams returns
Venus Williams also resisted a strong second-set challenge to overcome seventh seeded Russian Elena Dementieva 6-1, 7-6.
While the unseeded American has been building up an impressive number of victories this season, Dementieva is returning after an eight week break after suffering a stress fracture to her ribs.
"It was a very tough draw to play Venus in the second round," said the Russian.
"At the beginning it was very tough to get into the game because she was so powerful and aggressive."
It took me a few games to get into the rhythm so I'd be able to have a long rally and go to attack her as well."