Daniil Medvedev has become the first player to advance to the semi-finals of this year's ATP World Tour Finals courtesy of a straight-sets win over Alexander Zverev in the Red group.

The Russian had eased past compatriot and good friend Andrey Rublev in his opening battle and survived an early scare to defeat Zverev 7-6[7], 6-4 on Wednesday evening.

In the 18th top-level meeting between the pair, Zverev brought up two set points in the tie-breaker, but the German squandered one with an unforced forehand error before Medvedev saved the second.

Zverev would also be left to lament a wasted opening in the second set, as he failed to take a break point against Medvedev in the ninth game before losing serve while trying to stay in the match.

"It was a very tough match mentally. I started very well, I was playing well. He was kind of not there yet, but at one moment he stepped up his game and at this moment, for whatever reason, I started playing a little bit worse for a short period of time," Medvedev said on the court.

"I'm happy that I managed to regroup and play better at the end of the first set. The end of both sets was very tight... In the tie-break he had set point on his serve. I'm happy because this year I might have lost some matches like this. I'm happy to be able to win."

Medvedev has now made the knockout stages of the ATP Finals for the third time, having been crowned champion in 2020 and losing the 2021 final to Zverev before his round-robin exit next year.

The third seed could seal top spot in the group on Friday when he faces Carlos Alcaraz, who all but eliminated Rublev from semi-final contention with a 7-5 6-2 win over the Russian earlier on Wednesday.