Spain's Carlos Alcaraz suffered a debuting defeat at the ATP Finals in Turin on Monday after taking an early lead against Germany's Alexzander Zverev 7(3)-6, 6-3, 6-4, as he complained about the fast in-door surface afterwards.

The world No.2 missed out on the last edition of the year-ending tour due to injury, but he still retains the hope of becoming the youngest Finals champion since Pete Sampras in 1991, achieving this feat at the age of 20.

"I don't know why they put this surface at the end of the year," said Carlos in his post-match news conference.

"All the tournaments we have played on hard court are so, so, slow and then we come here to the Masters (Cup) and they put this court so, so fast.

"I don't understand why they've set the speed so high for this tournament when we have never played on a surface like this all year," he added.

"This tournament is different for many things, and one of them is that it allows you to go through to the semi-finals despite losing the first match," Alcaraz posted on his Instagram account as he is set to play against Andrey Rublev and Daniile Medvedev of Russia later in the group.  

Meanwhile, Medvedev downed good friend Rublev 6-4, 6-2 to improve their head-to-head record to 7-2.

Medvedev had beaten Rublev twice this year at the Dubai Championships final and the U.S. Open semifinal, and is on course to level Serbian Novak Djokovic and Alcaraz on six tour-level titles this year.

"First set was really tough. I managed to just get a little bit the upper hand on the most clutch moments, and this helped me in the second, so I'm really happy," said Medvedev who saved seven break points in his last two service games of the first set before taking his second-set victory.

Tuesday's two round-robin matches of Green Group feature Greek Stefanos Tsitsipas taking on Denmark's Holger Rune, and world No.1 Djokovic, who is seeking to set a new record for most titles at the ATP Finals, playing against Italian Jannik Sinner.