The Croatian revealed Alcaraz is yet to convince him on indoor courts, while he named Alexander Zverev as the player who is “closer” to Sinner in such conditions.
The ATP Finals, which features the eight players who collect the most points in the season, is played on indoor hard courts.
The 2024 edition of the year-ending championships will be staged at the Inalpi Arena in Turin from November 10 to 17.
Sinner, Zverev, Alcaraz and Daniil Medvedev have all booked their spots at the ATP Finals, while the other four qualifiers are still to be confirmed.
Alcaraz is yet to win a title or reach a final on indoor hard courts and he has found the surface more challenging than others in his short career to date. The four-time major champion has claimed eight titles on clay, five on outdoor hard courts and three on grass.
At the 2023 Paris Masters, which is the other big indoor tournament on the ATP Tour, Alcaraz suffered a surprise straight-set loss to Roman Safiullin in the second round.
The Spaniard was then crushed by an inspired Novak Djokovic in the semi-finals of the 2023 ATP Finals on his debut at the event. Alcaraz qualified for the ATP Finals for the first time in 2022, but he was forced to pull out due to injury.
Sinner, the world No 1, has claimed six of his 17 titles on indoor hard courts, while he was an ATP Finals runner-up in Turin last year.
Zverev, meanwhile, has won seven of his 22 career titles at indoor hard-court events — and he triumphed at the ATP Finals in 2018 and 2021.
Speaking to Sky Sport Italy, Ljubicic addressed whether Sinner is the favourite to triumph at the 2024 ATP Finals.
“Yes, for me, he (Sinner) is the dark horse,” said Roger Federer’s former coach.
“Also because Alcaraz still has to convince me on indoor courts. He has a very high level on hard court, but even in Turin last year, he struggled a lot on indoor courts, especially from a mental perspective.
“Let’s see if that changes this year. If we want to look for someone closer to Jannik, I would say Zverev.”
