Morikawa took over as World Amateur Golf RankingTM number one in May 2018 when he replaced Walker Cup teammate Braden Thornberry at the top of world order. The University of California player was worthy of the top spot considering his excellent amateur record. In March 2018, he won the Querencia Cabo Collegiate for his second collegiate victory of the season. That same month he played in the PGA Tour’s Arnold Palmer Invitational and finished joint 64th. He includes the 2017 Northeast Amateur and 2016 Sunnehanna Amateur Championships as victories outside college golf. He was a prominent member of the victorious 2017 U.S. Walker Cup team, so prominent that he was one of three players along with Doug Ghim and Maverick McNealy to earn four points out of four.