Mentioned up a few posts above as things against Nadal for this match and I'll add that he's not in full form like he was in Barcelona because of simply not enough hard court matches.
Things that might get Tsitsipas the win:
1. Has not been broken in 4 of his 5 matches at Rogers Cup.
2. Zverev broke five times a Tsitsipas who had a lackluster first serve and a low percentage to boot (47.5%). Redlining Zverev has a lot of reach on first serve and has been returning exceptionally well this Summer.
3. Very smart tennis player who will try to adapt to deal with Nadal changing tactics
4. Calm like a professional assassin who won't be phased by the Nadal celebrations
5. Incredibly sneaky forehand that should be able to hit through any short balls garnered from Nadal.
6. Great depth (a key to winning) that will be even more effective on hard courts
7. Nadals heavy shots actually get to the opponent slower which means Rafa will have trouble rushing Tsitsispa
8. Slow deep returns that will stop Nadal from mauling right after the serve
9. Nadal does not have a heater serve like Anderson, et al to expose the Tsitsipas first return
10. Nadal on worse surface for him than Barcelona
11. Tsitsipas playing a touch better than Barcelona (where he dismantled a lot of solid clay court players)
12. Khachanov gave Nadal a tremendous match until their first set tiebreaker which suggests Nadal can definitely be had.
Less obvious Nadal plus:
Tsitsipas is more of a clay court player than a hard court player. He's won a much lower percentage of points over the last two weeks than on clay in Barcelona, but this is tempered by the toughness of the field with Duckworth in Washington the only weak player faced over the last two weeks.
Predicting Nadal in 2 tight sets or possibly three. Hoping that Tsitsipas gets in a good set to set the stage for future clashes. Tsitsipas did this with Zverev in Washington with a near fifteen minute hold where he was ultimately broken for 5-4 in the 3rd. Amazingly he took the next step against a Zverev goating on return at Roger's Cup.
Hopefully TsitsiFast throws down a gauntlet today having us wanting more matches between these two on all surfaces.