Zverev beats Alcaraz in straight sets to reach last four of ATP Finals & knock Spaniard out
German Zverev came into the contest as the group leader with two wins from two matches - against eighth-seeded Russian Andrey Rublev and Norwegian sixth seed Casper Ruud - but needed to win at least one set against Alcaraz to book his place in the semis.
Third seed Alcaraz needed a win of his own and is now out of the season-ending tournament.
Even if Andrey Rublev, who has lost his last six Finals matches, beats Norwegian Casper Ruud in straight sets later on Friday, the world number three would still be knocked out on account of his game-winning percentage, which is guaranteed to be lower than that of whoever wins the final group-stage match.
With the win, second seed Zverev ensured a first-place finish in John Newcombe Group. He will next face US Open finalist Taylor Fritz.
The victory was even sweeter for Zverev, having lost the French Open final against Alcaraz in June.
"He (Alcaraz) beat me too many times this year in important matches, so I'm happy that I got this one," Zverev said.
"Obviously I feel like we have a pretty good rivalry with a great friendship as well, so it's always nice to play against him... Just sharing the court with him.
"He's a great guy and I'm looking forward to the next matches with him as well.
"It's been three fantastic matches. I'm happy with being 3-0 in the group for sure, but I think now the semi-final is going to be very difficult. (Taylor) beat me the past few times at the slams, so I'm looking forward to that match."
Zverev and Alcaraz failed to find consistency in a tight opening set as momentum swung back and forth before the German came out on top.
Zverev dominated for most of the second set, building a 3-1 lead before surviving a late fightback from Alcaraz.
Zverev won 73% of his points on serve and thumped 29 winners in the one hour and 57-minute contest, taking a 6-5 head-to-head advantage over Alcaraz.
It was the world number two's 69th win of the season, the most victories in one season since three-time major winner Andy Murray (78) in 2016.
Since reaching his first semi-final at the ATP Finals in 2018, only record 24-times Grand Slam champion Novak Djokovic (five) has made it to the semis of the tournament more than the German, who has booked his place four times.
Alcaraz, who picked up his first win at this year's ATP Finals when he beat Rublev in straight sets on Wednesday, again wore pink nasal tape, having suffered with respiratory issues in recent days.