The American ended the day on seven-under 66, for a total of 16-under 130.
It is a two shot lead in front of Masters Champion Scottie Scheffler and 86th-ranked Spaun.
— PGA TOUR (@PGATOUR) January 7, 2023
Rahm dropped down the leader board to a share of sixth alongside England’s Matt Fitzpatrick on 11-under 135.
American Morikawa finished last year win-less but fought his way into a shared lead on the first day with six straight birdies before pushing ahead of the pack in the second round.