Ben Shelton, Sunday's Tokyo champion, rounds out the quartet at No.15.
The Dane hopes to work his way out of a tailspin and into the Turin field.
Meanwhile, Daniil Medvedev will bid for a title defense in Austria.
The eight-team field is set for the WTA Finals, which commence on October 30 in Cancun, Mexico
"If I set dates and I don't meet them, I'm adding disappointments that I don't need," said Rafael Nadal of his comeback aim.
World's top two to join Aryna Sabalenka and Ons Jabeur playing Riyadh Season Tennis Cup in Saudi Arabia.
The powerhouses will convene on Sydney and Perth in late December.
