Healthy and fit del Potro, who had an easy draw including the Nadal retirement so was fresh for the final, isn't easy to beat in straights (save AO) no matter how insipid it looks at first glance. Also BS on not playing a great semi, actually never got broken, didn't even face BPs iirc, quite impressive against Nishikori who is strong on return, obviously more than Nadal denying BPs to the likes of Anderson and Berrettini whose return game is significantly worse (service game better but not relevant to the point).