While Alcaraz continued his stroll through the draw with another routine straight-sets victory over Czech Jiri Lehecka, Djokovic had to survive a gruelling quarter-final against American fourth seed Taylor Fritz to keep his dream of a record-setting 25th grand slam crown alive.
At 38 and playing against a rival more than a decade younger, Djokovic came out on top 6-3 7-5 3-6 6-4 after a three-hour 24-minute battle littered with lengthy and lung-busting rallies at Arthur Ashe Stadium.
The four-time Open champion conceded he didn't know how he'd won.
"Incredibly close match. It was really anybody's match," Djokovic said after dedicating victory to his daughter Tara on her eighth birthday.
"I was lucky to really save some crucial break points in the second set. For most of the second and third set, he was a better player.
"And in this kind of matches, a few points decides the winner. It was fortunately coming on my side, particularly the end of the fourth.
"That last game was nerve wracking."
Set for his 53rd grand slam semi-final, it won't get any easier for the four-time US Open champion against Alcaraz.
The second seed will be looking to exact revenge after losing to the mighty Serb in the Paris Olympics gold medal match and also in this year's Australian Open quarters.
The Spaniard is yet to lose a set at Flushing Meadows this year and there was never any likelihood of that changing against Lehecka, with Alcaraz needing only an hour and 56 minutes to see off the 20th seed 6-4 6-2 6-4.
He celebrated victory with a golf swing before telling the watching former US Masters champion Sergio Garcia in his courtside interview he was looking forward to playing a round with his compatriot on Wednesday.
Not every tennis pro would play a round of golf the day before a grand slam semi-final but Alcaraz is focused on ensuring his downtime relaxes him for court duty.
And it seems to be working.
He is seen as not as consistent as his big rival Jannik Sinner but he has not lost before the final of any event since Miami in March.
His shock second-round defeat here 12 months ago, meanwhile, is the only time in his last 12 grand slam appearances that he has failed to reach at least the quarter-finals.
Lehecka can claim a win over Alcaraz this season, in Doha in February, while he also pushed the 22-year-old in the final at Queen's Club, so there was reason to believe this might be a real test.
But the Czech's big serve and hard, flat groundstrokes were no match for the ingenuity and X-factor possessed by Alcaraz, who even impressed himself with a forehand winner thumped down the line from the backhand corner to win the second game.
By that point he was already a break up, and it was all Lehecka could do to try to stay within touching distance.
The Czech did not manage to force a break point in the match and any glimpse Lehecka had was inevitably snuffed out by Alcaraz magic, such as the lunging drop volley at 15-30 in the sixth game of the second set, the five-time grand slam champion putting his finger to his ear as he soaked up the cheers.
With AAP.
8°C