Some quotes I've gathered about Jay's AS from various sources and which I posted on my Jay blog:
""Jay's worst affliction is his back, because of his ankylosing spondylitis. The pain can wake him up and he has to sleep sitting up. Initially, I got a shock."----- Xiao Mai, Jay's good friend and choreographer
From his mother's foreword in his book, Grandeur de D Majeur (circa 2003) comes this indication of how severe his pain could be: (Translation taken from Jay Chou Studio)
"Jay who doesn’t smoke, doesn’t drink, doesn’t go to night stalls, the thing that worries me the most is his old illness - ankylosing spondylitis, sometimes when I see him in so much pain, even when he gets up from bed to go to the door, he takes 20 minutes, being his mother, I could only shout in my heart: “God! Can I take the pain for him?” My son seeing my reluctance, instead he comforted me: “Mom, I’m alright, I’ll be fine in a moment.”
And from the Jay Chou Studio archives, here's a quote about his back pain circa 2009, which also bears testimony to Jay's strong determination not to let AS get the better of him and that there are others who are much worse off than him:
" I still think I'm very lucky, no matter how much more it hurts, someone else will be hurting more than you, no matter how much more pitiful you feel, there's someone more pitiful."
I also found a good snippet from an interview during his promotions for Treasure Hunter where he talked about his AS:
"I'm not afraid of getting injured. I only worry that my back will act up when I have to film a scene. Even when I have concerts, I always hope that my back won't act up because I have to dance or perform. That's a constant worry I have. whether I'll be okay when I have to work. In Treasure Hunter, I was fairly lucky, my back only hurt for a couple of days. It's a good and bad thing, this back problem. Because of it, I didn't have to do national service and I got to do the things I wanted to do. I got to become a singer. I can't say I'm thankful but if it weren't for this problem, I may have gone into national service and then maybe I would not have become Jay Chou. So no matter how difficult it is, I accept it."