It's an interesting development in the season because nobody really expected it. The dramatic and sudden rise of Alcaraz surprised many including himself as he had the goal of being inside the top 15 at the end of this year. Possible being number is crazy development for the young Spaniard. If he gets it, he would become the youngest number one ever which is quite something.
In the live rankings, Nadal currently leads but he won't get that number one spot if Alcaraz and Ruud get to the final. They both need to make the final to have a shot at being number one. If Alcaraz goes to the final he'll be ahead of Nadal for sure but he must make sure that Ruud loses in the final to him or that Khachanov beats the Norwegian in the semi-final.
For Ruud, it's the same thing. If he gets to the final he'll overtake Nadal for sure but he needs to beat Alcaraz to get it if the Spaniard makes the final. Alcaraz plays Sinner later today so if he loses, only Ruud will have the chance to become number one and he would only need to beat Khachanov which seems likely. See the breakdown below.
Race to Number 1:
*Total number depending on final placing in the event