In the first headline clash at this year’s season-ending event in Turin, defending champion and world No2 Medvedev edged past third-ranked Zverev, the 2018 winner, 6-3 6-7 (3) 7-6 (6).
The Russian, who claimed his maiden Grand Slam title at the US Open in September, has now won the last five meetings with his German rival and leads their career head-to-head 6-5.
Zverev has also enjoyed a stellar season, the highlight of which was a gold medal at the Olympics, but Medvedev clearly has his number at the moment.
Their last meeting came just 10 days ago in Paris, a 6-2 6-2 triumph for Medvedev.
This was a lot closer, although Medvedev was in total control of the first set after securing a break in Zverev’s opening service game.
The 25-year-old was irked by a marginal foot-fault call which cost him a point in the second-set tie-break, sarcastically giving a thumbs-down gesture to the crowd, as Zverev levelled the match.
But in the third Medvedev saved a break point for 6-5 and came from a mini-break down in the decisive tiebreak to make it two round-robin wins out of two and move to the brink of the semi-finals.