Alcaraz, who a couple of days ago was in danger of facing group-stage elimination from the tournament, picked up a pair of round-robin wins over Andrey Rublev and Daniil Medvedev to turn his fortunes around. He also snapped his three-match losing streak in the process.
The Murcia native eventually topped the group ahead of Daniil Medvedev to set up an enticing semifinal clash against reigning champion and World No. 1 Novak Djokovic - the fifth installment of the most riveting rivalry in men's tennis right now.
By reaching the semifinals, Alcaraz did something that Rafael Nadal accomplished 17 years ago when he was also 20 years old. The man from Manacor compiled a 2-1 group stage record at the ATP Finals (formerly Tennis Masters Cup) which was held in Shanghai, China, at the time.
Like Alcaraz, Nadal also lost his opening match at that year's season finale to American James Blake but defeated Tommy Robredo and Nikolay Davydenko to qualify. Nadal, ranked World No. 2, faced rival Roger Federer, who was the No. 1 in the semis and lost in straight sets.
Alcaraz will be hoping the outcome in the semifinal is different from that from 2006 when he faces Djokovic on Saturday. The Spaniard lost the battle for the year-end No. 1 ranking to the Serb, and a defeat at the ATP Finals would be a double whammy.
But there is a list that Alcaraz sits ahead of Djokovic and the rest of the field, and that is the ATP bonus pool for the ATP 1000 and year-end championships. He will need to defeat Djokovic to maintain his lead over Daniil Medvedev and Jannik to earn the bigger share of the $20 million bonus.