Novak Djokovic begun his ATP Finals campaign with a straight set victory over Stefanos Tsitsipas in Turin, ending the Greek's hopes of finishing the year ranked number one.
The five-time champion emerged as a 6-4 7-6 (7-4) victor to go top of the Red Group above Andrey Rublev, the Russian having defeated compatriot Daniil Medvedev earlier on Monday.
Tsitsipas needed to win the title in Turin without losing in order to dethrone the injured Carlos Alcaraz at the top of the rankings, but now cannot catch the Spaniard.
Djokovic did not face a break point as he served superbly in the opening set, not allowing the second seed a look-in after breaking in the first game of the match.
Tsitsipas responded well in the second set, firing down seven aces and winning 88 per cent of points behind his first serve, but seventh seed Djokovic stood firm to take the set to a tie-break.
A battling Tsitsipas fought back from 5-1 down to 5-4 in the breaker, but Djokovic did not falter, coming up with a powerful serve that his opponent was unable to return to make a winning start in his quest for a record-equalling sixth ATP Finals title.
Tsitsipas - 9/0
Djokovic - 3/2
Tsitsipas - 24/0
Djokovic - 33/2
BREAK POINTS WON
Tsitsipas - 0/1
Djokovic - 1/2