Hall of Fame
So I looked into some of his second serving in 3-4 sets of the final, which he actually made. They were 83-87 mph, which is very slow for such a server - already slowing down. They were not kicking high either for such a big server - barely 6 ft.Interested to see what you come up with.
What stands out for me, in addition to all previously posted here and elsewhere by big minds, is how his toss for second serve is too straight up (or how he doesn't get under the toss), how his body launch is straight up, and, consequently, how his contact is at the zone where racquet is already barely rising - too high, if you like. Two examples:
Compare to how Thiem does it (at faster pace);
Sascha basically sets up with his toss and trophy very similar to his first serve, and ends up lacking topspin - he cannot shape the ball to dip down. Spin he produces staying sideways and swinging across is mostly sidespin. Hence his margin for error - window to hit over the net and into the box - is not bigger compared with 1st serve if he hits at around 100 MPH! He only gets higher curved trajectory by taking major pace off the ball. Of course he can land many slice second serves when his 100% on, but that's exactly the case - not reliable for tough situations and pressure.
I have yet to find good second serves which are not repeated 1st serve attempts, will look through 1-2 sets. But I bet those are technically same - lack topspin and up-down shape.
Now still an open question how to put the toss reliably to where it should be - quite possible we're back to square one - get it lower, while also simplifying/speeding up the prep.