How do I respond to this attack?

My tall friend and I like to spar every so often. He's a powerful guy and he likes to come at me with... I guess they're like closed-fist chops... he brings one hand up and to the side then sort of hammers me. I find blocks pretty ineffective against these, and I realize he leaves his body completely open setting up for these, but I'm not fast enough to get in and out of his range in time to avoid the strikes. Any suggestions?