Sinner essentially controlled the fate of Djokovic as he could have technically tanked the final match, which would have eliminated the Serbian, sending himself and Rune to the semi-final to face Daniil Medvedev and Carlos Alcaraz.
Now, the incentive to win for Sinner was obviously the money and the points that the players get with every win, so he was playing for a sort of reward even if the match didn't matter too much in the grand scheme of things.
Sinner would beat Rune, allowing Djokovic to continue at the event and ultimately win it. The Serbian bested Alcaraz in the semi-final comfortably and then also Sinner in the final to win the event, which makes this a really fascinating what-if scenario.
Some have already argued that Sinner should have tanked that match to eliminate Djokovic, but he took the high road. He just kept winning wanting to prove himself as a true champion, and he did, despite going home with the smaller trophy.
After all, that match against Rune was quite complicated. Rune was pushing hard and even had a break ball in the final set of the match, but Sinner wouldn't let up. He dealt with some back issues as well but, again, wouldn't let up, and that's a proper champion mentality.
He wanted to win that match and then face whoever he was supposed to face. Unfortunately, he ran out of steam in the final but earned the respect of many. And he'll have more chances in the future.