Hitting 1st serves as your 2nd serves is a viable tactic. If you can hit 70% and win more than 70% of those points, you are winning around 50% (or more) of those points, which is winning a good stat for 2nd serve points. I remember some old article discussing that some players should serve two 1st serves. Anyway, if you do it all the time, you might lose the surprise element.While Medvedev played hit and hope tennis and will never do that sort of thing again (130mph 2nd serves is desperation)
A loss here and there might only serve Djoker well. Medvedev on the other hand, has won too much already on this HC trek, will be taken out by some young hot gun on a good day. Nole is still very much the favorite for US Open. Nadal is the 2nd, but could well get injured etc. Federer has not played much, and has not won much, this year.