World number one Jon Rahm's Ryder Cup prep week hasn't gone quite according to plan, but the Spaniard laid low by a stomach ailment said he was improving after an even par 72 in the first-round of the US PGA Fortinet Championship.

Rahm withdrew from Wednesday's pro-am with the bug, which he attributed to fatigue after battling to a runner-up finish to Patrick Cantlay in the US PGA Tour Championship last week.