US Open

The International Tennis Federation's 2018 Official Grand Slam Rule Book stipulates that players can request permission to leave the court for a "change of attire break" to change their clothing at the "nearest assigned bathroom." Hard to predict what scene will be for GA seats at Armstrong and GS… They are definitely releasing more grounds passes this year, given the increased size of Armstrong, so it might end up being similar to previous years… But regardless, with your Ashe ticket you can enter grounds as early as 9:30, head to an outer court for an 11am match that appeals, then enter Ashe whenever you want (Ashe matches will start at Noon), then bop around as you please… P.J. If it were me, would probably use them for the day session, which tends to be more expensive (and when you can use them to see play on non-Ashe courts too), then devote your max budget to good evening Ashe tickets.
Last year, for instance, I had reserved seats in Ashe during R16 while Federer was playing and also had amazing seats in Grandstand while one of the most exciting matches was taking place with Del Potro coming back after losing the first 2 sets… The Grandstand crowd was cheering so loudly every few minutes that we could hear it in Ashe, and even as I watched Federer from great seats I was experiencing FOMO”. Go early (8:30am if possible), get in line, and based on schedule released the prior day go directly to grab general admission seats in whichever stadium (beyond Ashe) has matches that appeal the most (probably Armstrong or Grandstand). I purchased tickets for Sat Sep 1st Evening session at Louis Armstrong for the three of us (seatgeek purchase, row L). I also purchased loge tickets for Arthur Ashe for Monday 3rd, evening session too (Ticketmaster, standard price, section 133, row L). I'm yet to purchase tickets for Sun 2nd and Tuesday 4th (quaterfinals).

If you want to supplement your tickets with one session with better seats in the Loge section, you might want to do that on Monday during the day session as prices are on average less because many tourists leave so demand is a bit less (even though matches are just as good). You will definitely see big name players in singles matches on Ashe during the Wed Day session. My plan for the daytime would be to pick up promenade tickets for Arthur Ashe so that I have access to other matches in the daytime.

Normally I recommend getting tickets in advance to avoid lines and to have total visibility into the options - especially if you see standard (non-resale) tickets available on Ticketmaster , as is the case for that Monday day session (there are Promenade seats going for as little as $70, Loge for $130, and even Courtside for $380). I got the finals tickets set, but I wanted to experience the most out of the US Open since it's going to be my first time there, so I'm thinking about going to see the QF match on Sep 5 (unfortunately that's the earliest I'm able to fly in) as I wanted the opportunity to see the Grandstand (GS) and Louis Armstrong (LA) stadiums too.I'd probably lean towards going on Saturday Sep 2 Day session (assume you meant Sat Sep 1 or Sun Sep 2) because it's still third round of play and there will be more matches happening on all the outer courts than on Sunday or Monday (when it's down to the last 16 singles players).

(1) If Federer is scheduled to play on Monday on Ashe for the session you have tickets for, you're all set. Thanks to your incredible tips, I splurged on courtside tickets for two days last year at Grandstand to watch my fav player Delpo play over labor day weekend (and yes, I was in the third row during that incredible Delpo-Thiem match). Aside from the 2 matches each in Ashe and Armstrong on the first Monday night is there usually a lot of stuff going on in the grandstand, court 17, outer courts, and/or practice courts after 6pm?
In case it impacts your recommendation for 1st timers, we'll probably get tickets for a few days (Ashe, Armstrong and/or Grandstand). I have one courtside ticket for Ashe day session on Friday 31st and I was expecting I can watch either of those 3 players