The Spaniard needed to win this match to find himself in the semi-finals, and he delivered a really fine performance. The win condition for Medvedev was always going to be the serve, and the better he served, the more likely he would be to win.
He didn't manage to create much separation in the opening set that way, hitting only one ace compared to three by Alcaraz. If you're getting outserved by Alcaraz, then you're in trouble, and Medvedev was.
Even in rallies, he couldn't quite assert himself properly. He was pretty precise, having only one unforced error, less than Alcaraz, but the crucial rallies still went the Spaniard's way.
He used his only break chance while Medvedev wasted two, and that's all it takes in close matchups like these.
The second set saw Medvedev serve a bit better, but he missed more in the rallies, so we still had this really close and competitive affair. Medvedev even had to fight off two early break points while failing to create any on the serve of the Spaniard.
It just didn't seem like Medvedev had any way of breaking through, and ultimately, he never did. Alcaraz broke at 5-4 to set himself up to finish the match, and it was the perfect timing.
He would finish it off in two sets 6-4 6-4 to secure his maiden ATP Finals semi-final appearance. What a brilliant performance by Alcaraz, and at what moment.
Alcaraz finished with three aces and 22 winners, which was a better mark than Medvedev's 3 aces and 14 winners. The Russian simply never looked comfortable in the matchup, but he shouldn't worry too much, as he's also through to the semifinals.