Medvedev looked on course to defend his Nitto ATP Finals title on Sunday evening, having beaten the German 6-3, 6-7(3), 7-6(6) in the 'Red Group' play of the WTA Finals, but he was at a loss to explain why his service deserted him in the title clash.
"It's tough to say, maybe some tiredness of the body, maybe mentally I wasn't 100 per cent," Medvedev told atptour.com. "Not that I didn't want to be, but definitely something was missing. I don't have the answer, but it made the difference.
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"Even when (my serve) was going on the line, it didn't really have that spark. It wasn't enough for Sascha (Zverev), who is a great player and broke me two times. Sometimes, in a way, it's not bad, but when you're playing in a big final on a fast surface against someone who is serving like Sascha, it's enough to win the match. We can talk about many things, but the serve was definitely the key, and he was better," said Medvedev.
Medvedev slammed 59 aces and won a tournament-leading 61 per cent of his second-service points in his five matches at the season finale, but the Russian couldn't beat Zverev for the third time in as many weeks.
"We all try to do something different, to prepare differently or play differently," said Medvedev, who had also beaten Zverev 6-2, 6-2 at the Paris Masters. "I felt he was doing something differently. Maybe I lost the first break of serve (at 1-1 in the first set) because of this, but at the same time if you make aces you get into the match.
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"I tried to change up things I'd been doing, and I felt it was working well, but I just couldn't return his serve and it was enough for him to win the matchï¿½ Every time you know he is going to do something new and you have to be prepared for it and the same for them. This time he won, but next time I will try to do something better."
Despite the defeat, Medvedev will finish the year as No. 2, having assimilated a 58-13 match record and won four trophies.
Medvedev also believes that Zverev could win a major soon given the kind of improvement he has shown. "Sascha is a great player who is capable of beating anybody, so he definitely can win a Grand Slam, because it's just obvious."