One time I was sparring a dude who proceeded to whup me so badly that at one point, after the fight went to the ground, he stood back up but I just stayed on the mat curled up in the fetal position. He started lightly kicking me, and saying things like "Come on, get up. The fight isn't over yet, you can't quit." Some people might think he was being too rough on me, but do you know what I did...I picked myself back up and I kept fighting until the buzzer went off.
I think it's good to experience things like that sometimes, because it really shows you that your limits are farther than you think they are. Usually when your mind tells you it's time to quit, it's a lie. Your body is able to withstand much more than your brain thinks it can. In the dojo I've heard it said that you haven't really worked out until you've either thrown up or passed out. That is the point when your body really can't take any more, and it usually occurs quite some time after your mind wants to quit.
I've thought I was going to pass out many times while training, but it hasn't happened yet. I've been so exhausted that I could no longer form rational thoughts, but my body still worked just fine. The people who are able to perform what seem like impossible physical feats are those who have mastered their minds as well as their bodies.
I have thrown up a few times while sparring...but not on the day I mentioned above. I had not yet reached my limit that day, and I'm glad my sparring partner did not let me quit.