Sinner now chooses who he wants to go through.
If he loses to rune, Rune will go through.
If he wins against Rune, Djokovic will go through.
It’s also worth nothing that Sinner will finish 1st with a win tonight but 2nd with a loss to Rune, not that it matters too much at this point considering we don’t even know who will top the other group… but he still has something to play for technically.
He can’t really choose though can he? Like Hubi can’t really “choose” to beat Novak here. It’s not a given, Rune leads the H2H 2-0 against Sinner.
Yes in terms of form Sinner is probably the best in the world right now since Beijing, but it’s not a given he manages to beat Rune, a Runs who’ll be motivated to win and knows he has a margin of error too and can go to 3 sets.
Things to consider:
Sinner has never won against Rune (2-0 h2h) and might want to even things out slightly.
Sinner will not want to play Djokovic again. Because Djokovic is Djokovic. And probably won’t lose twice to the same guy in the same tournament.