Dustin Johnson admits he has work to do to maintain his status as world No 1, starting at the AT&T Pebble Beach Pro-Am in California.
The former US Open champion has topped the world rankings since winning the Genesis Open last February, but could see Jon Rahm overtake him if the Spaniard claims a third win in six events.
Johnson claimed an eight-shot win at the Sentry Tournament of Champions and posted a top-10 in Abu Dhabi, with the two-time Pebble Beach champion looking to replicate the form that saw him win three consecutive events during the same time of year in 2017.
"It's just how it goes," Johnson told a pre-tournament press conference. "If I keep playing well and keep winning then I'm going to stay number one. If I don't play well, there's a lot of really good players that are playing really well right now.
"So it's going to be tough for me to keep it, but if I keep playing like I am and how I should, then, yeah, I'll be there at the end of the year."
Johnson will play alongside defending champion Jordan Spieth for the first three rounds, while Rory McIlroy - making his Pebble Beach debut this week - tees off with Phil Mickelson.
The 33-year-old was a back-to-back winner of the event from 2009 and finished third in last year's tournament, and Johnson is relishing the chance to return to the venue.
"I just really enjoy coming here," Johnson added. "I enjoy the courses (Pebble Beach, Spyglass Hill and Monterey Peninsula), they're a lot of fun to play.
"This event is a special event. You get an amateur partner, you play with one other pro and his partner, it's just a lot of fun."
Watch the AT&T Pebble Beach Pro-am throughout the week live on Sky Sports. Live coverage begins on Thursday from 4pm on Sky Sports Golf and Sky Sports Main Event.