The world ranking number-crunchers suggested on Saturday that Koepka needed to finish no worse than a two-way tie for 11th at the Dunlop Phoenix tournament in Japan to remain on top.
The American, however, finished 12th. One lousy shot better and he would have tied for 10th and remained number one.
The updated rankings are projected to have Rose on 10.0344 points, and Koepka on 10.0332 points.
Rose took the week off, while Koepka was defending his title in Miyazaki.
Koepka and Rose have swapped world number one with monotonous regularity over the past month.
Koepka gained the top spot four weeks ago, and held it for a fortnight before handing it off to Rose.
Rose stayed number one for only one week, before handing it back to Koepka, who after one week has given it straight back to Rose.
(Reporting by Andrew Both in Tokyo; Editing by Amlan Chakraborty)