The Spaniard Jon Rahm has since the turn of the year won three of the six competitions he played, and came third in a fourth.
But in the big competition The Players Championship, which is often called "the fifth major", he got off to a tough start.
He was only tied for 32nd place after the first round, and heading into the second round he announced his withdrawal from the competition.
The reason was illness, the PGA Tour announced on Twitter.
Scheffler can take first place
Rahm is now at risk of losing first place in the world rankings.
If Scottie Scheffler finishes in the top five or Rory McIlroy in the top eight, Rahm loses his place as world number one.
American Chad Ramey, ranked 225 in the world, led after the first round at eight under par - seven strokes better than Jon Rahm.