Carlos Alcaraz was beaten in three sets by Alexander Zverev on Monday as the world No 2 started his ATP Finals campaign in disappointing fashion.
Alcaraz, making his debut at the season-ending event, took the first set after a tiebreak in Turin but seventh seed Zverev fought back to level the scores before going on to seal a 6-7, 6-3, 6-4 win.
The Spaniard went into the match already knowing there would be no chance of overhauling Novak Djokovic as world No 1 before the end of the year after the Serbian defeated Holger Rune on Sunday.
Alcaraz was knocked out of the Paris Masters by Russian qualifier Roman Safiullin in his first game back from injury having not played since the Shanghai Masters in early October due to lower back and left foot problems.