Yamato's existence is kind of a contradiction. We see that he has a stat of 2 on everything, but it also says his full power is hard to gauge. Well the only other two persons who have powers that cannot be easily gauged are Oni and Byodouin. Besides, how would Yamato's games against say, court 4 work? If he plays like a guy with all 2s in his stats he'd get crushed, but if he can actually beat a guy on court 4 then it'd be obvious his stats is above 2 across the board, so does he just never play a game after he got to court 3 somehow?
By the way Yamato wasn't playing toe to toe without Tezuka without GUYU. He was getting crushed which is why he had to use GUYU. Based on the manga, it'd appear GUYU only works on people who are extremely sure about their predicative abilities. After all, the second door is pretty much just an extension of data tennis. In fact, Tezuka's predicative abilities can only work if he is 100% sure of his predictions. It'd be the same as say when Inui played Yanagi and Inui said like '90% chance to serve to side' but Yanagi served to the middle instead and caught him off guard, except in Tezuka's case it's implied he is 100% sure of his prediction so he doesn't even think of the possibility his prediction can be wrong, which is why GUYU is devastating on him. For any normal person, since they're never 100% sure of their prediction to begin with, they'd be less affected by GUYU.
Of course there's also the fact that despite claiming he's going to retire after playing Tezuka, his name was on the list of the guys who are going to play the 1st stringers and, in particular, the G10. In fact I don't think any other guy from the former court 3 was scheduled to play the G10, so not counting Irie, he's apparently a lot stronger than the rest of the 3rd court for some inexplicable reason.
---------- Post added at 03:58 PM ---------- Previous post was at 03:51 PM ----------
Actually, after thinking about this some more, there are 7 guys in court 1 including Tokugawa (Tanegashima obviously can't play 1st string because he's 1st string) plus Irie and Oni, so that means Yamato is the strongest high schooler that isn't on the 1st court since he's part of the 10 guys playing the G10 originally.