Nadal because his two main competitors probably won't play...while Andreescu still have to deal with people Serena, Kenin, Osaka and a whole bunch of other players that live in the USA...most top players in the men don't live in the US and probably won't be there....
Andreescu has been injured since the USO and did not compete at the AO, so is a bit of an unknown quantity.
She was pretty awesome 😎 last year.
Nadal will probably win the USO for a record equalling 5th time if his main competitor doesn't show up.
Rafa is 2-1 vs. Djokovic in their US Open meetings, and it would be 3-1 if they'd met at 2018 US Open but Rafa got injured and retired in the SF.
After all, their 2018 Wimbledon Final was played with a closed roof, and Djokovic still needed 10-8 fifth set to win!