Rafael Nadal's decision to end his season early means Cilic is actually holding the last berth for the eight-man showdown at London's O2 Arena.
Marin Cilic beat Japan's Kei Nishikori for the Basel Open title on Sunday to edge closer to securing his place in the ATP World Tour Finals in London next month.
The powerful Croatian won 6-1, 7-6(5) to claim his 16th ATP Tour title, and consolidate ninth place in the Race standings.
Rafael Nadal's decision to end his season early means Cilic is actually holding the last berth for the eight-man showdown at London's O2 Arena.
Cilic is now 210 points above Czech Tomas Berdych with Belgium's David Goffin a further 190 points adrift.
He needed only 25 minutes to take the opening set as he overpowered world number five Nishikori who is already assured of a third consecutive appearance at the season-ender.
Nishikori offered more resistance in the second set and had three set points at 5-4 but Cilic held firm and claimed the title by winning the tiebreak on a Nishikori double-fault.
Former US Open champion Cilic dropped only one of his 54 service games on the way to the title, his second of the year after also winning in Cincinnati.
Cilic will also lead Croatia in the Davis Cup final against Argentina next month.
