User blog comment:B214/High School DxD DX.4 discussion/@comment-30924344-20170918222903

Confirmed by n0m@n: Saji's younger sister was there at his match cheering him on and knows about the supernatural world however, his younger brother wasn't there as he is too young to learn of the supernatural world.

So the main reason Saji could fight so long against Issei is because is younger sister was there supporting him and of course he no doubt had his younger brother in mind too.