ROME -- It's not just Grand Slam titles that Jannik Sinner and Carlos Alcaraz have taken turns collecting this year.
The top two tennis players are also trading the No. 1 ranking back and forth.
Sinner officially returned to the top spot in the ATP rankings on Monday following his Paris Masters victory - replacing Alcaraz, who had held the honor for nearly two months.
But due to the rankings' math and rules, Alcaraz will return to No. 1 next Monday.
The Italian and the Spaniard are separated by so few points now - Sinner leads with 11,500 points to Alcaraz's 11,250 - that it will come down to their performances at the upcoming ATP Finals in Turin to determine who ends the year at No. 1.
Points dropping off While neither Sinner nor Alcaraz are playing this week - except for when the ATP Finals start on Sunday - the reason the rankings will change again next Monday is because that's the day when the points from each player's performance at last year's finals drops off.
Sinner next week will lose the 1,500 points he earned from winning the finals last year, while Alcaraz will lose only the 200 points that he earned after failing to advance from the group stage.
So next Monday, the rankings will read: Alcaraz 11,050 and Sinner 10,000.
And there's zero possibility for any other player to move into the top two spots anytime soon since No. 3 Alexander Zverev (5,560 points), No. 4 Taylor Fritz (4,735) and No. 5 Novak Djokovic (4,580) trail far behind.
Sinner and Alcaraz have amassed their huge points totals by winning - among other tournaments - all four of this year's majors.
Sinner won the Australian Open. Then Alcaraz beat Sinner in an epic French Open final. Sinner got some revenge by beating Alcaraz in the Wimbledon final. And then Alcaraz beat Sinner again for the U.S. Open trophy.
ATP Finals seedings Making matters more complicated is that Alcaraz will be seeded No. 1 for the finals - even though he won't be ranked No. 1 entering that tournament.
That's because the finals uses another rankings system - the "Race to Turin" that takes into account only points accumulated in the calendar year.
Alcaraz leads Sinner by more than 1,000 points in the Race rankings.
Sinner lost ground in both systems during a three-month doping suspension near the start of the year.
Year-ending scenarios Sinner needs to win the ATP Finals and hope Alcaraz doesn't reach the final to secure the year-end No. 1 for a second consecutive year.
Alcaraz needs to win three matches at the finals to finish 2025 atop the rankings for the second time in his career. Alcaraz became the youngest player to finish a year at No. 1 when he achieved the feat as a 19-year-old in 2022.
As for the WTA rankings, Aryna Sabalenka has already secured the year-ending No. 1 spot.
Sinner enters Turin on a 10-match winning streak after titles in Vienna and Paris, will be playing on his favorite surface - an indoor hard court - and can expect loud support from his home fans at the finals.
Alcaraz goes to Italy coming off a loss to Cameron Norrie in his Paris opener and has often struggled when playing indoors.
Auger-Aliassime, Musetti still trying to qualify The other players to qualify for the finals - which features the top eight of the Race - so far are: Zverev, Djokovic, Ben Shelton, Fritz and Alex de Minaur.
Paris finalist Felix Auger-Aliassime and Lorenzo Musetti are still battling for the final spot, with Musetti needing to win a tournament in Athens this week to move ahead of the Canadian and qualify for the elite event in his home country.
It's also unclear, though, if Djokovic plans to play the finals at a point in his career where he has said he's focusing almost exclusively on the majors.
Djokovic, who withdrew from the finals last year, is playing in Athens this week.
If Djokovic withdraws, both Auger-Aliassime and Musetti would qualify for the finals.
The draw for the finals is slated for Thursday - when it could still be uncertain who the last qualifier is.