Spieth is the fifth American to ever reach No. 1 - joining Fred Couples, Tom Lehman, David Duval and Tiger Woods. At 22, he is the second-youngest player to ever reach the top spot, behind only Woods, who did it at 21.
As for Sunday's champion, Day is now the third-ranked player in the world, his highest ranking to date.
And in our weekly Tiger Woods watch, the former world No. 1 dropped from 278th to 286th with his early exit in Wisconsin. He sits sandwiched between Brazil's Lucas Lee and South Korea's S.J. Park.
Here is your new top 10, with a new No. 1, for the week of Aug. 16: 1. Jordan Spieth, 2. Rory McIlroy, 3. Jason Day, 4. Bubba Watson, 5. Justin Rose, 6. Jim Furyk, 7. Dustin Johnson, 8. Rickie Fowler, 9. Henrik Stenson, 10. Sergio Garcia.