Both semi-finals take place on Friday in New York, where third seed and favourite Alcaraz faces rising star Frances Tiafoe, while Ruud will lock horns with Karen Khachanov.
Should Alcaraz and Ruud tumble out of the final four, Nadal - who was upset by Tiafoe in the quarter-finals - would return to the top of the ATP Rankings for the first time since January 2020.
However, if Alcaraz and Ruud move through to the final at Flushing Meadows, to the victory belong the spoils. Either way, the young duo are set for career-high ATP Rankings after the final Grand Slam of the year.
"I don't want to think too much about it, honestly," Ruud told atptour.com about the race for No.1.
"It's something that of course all young players dream about, so if I'm in a position to do it, let's see if I can accomplish it. Of course, it's a little bit of extra motivation to dig in, and even if you're down on the score, to keep fighting. You never know what's going to happen and if I'm very, very lucky I can leave New York as World No.1. So I'm trying to go for it of course."
Alcaraz was asked about the mantle following his five-set, five-hour epic over Jannik Sinner. Understandably, the young Spaniard was just focusing on being in the moment...
"I just was thinking about the match, not the ranking or results or stuff," said the 19-year-old.
"I just try to be focussed, try to put my best in every shot, in every game."