If there was a chance that he could defeat Rider, then perhaps he would not reject the use of his Noble Phantasm. However, Rider was effectively immortal, and Saber could not imagine that his Noble Phantasm would work against him.

Rider's protection is not something that can be pierced by raw power. Something more is needed. It could be that one needed to use fire or lightning against him. It could be based on certain conditions; perhaps Rider was nigh invulnerable within a forest, or at night.

- Vol 1 Chapter 4