One of Jason Day's goals at the start of the year was to stay at No. 1 for the entire calendar year. The year is not even two months old, and already he is facing his first big challenge.

And not just from one player.

Dustin Johnson, coming off a third-place finish at Pebble Beach, and Phoenix Open champion Hideki Matsuyama both have a mathematical chance to reach No. 1 in the world by winning the Genesis Open at Riviera. Johnson would need Day to finish around fourth or worse, while Matsuyama could reach the top only by winning and Day finishing about 25th or worse.

Tiger Woods in 2009 was the last player to start and finish a year at No. 1.

Rory McIlroy was the last player to stay at No. 1 for longer than a year, starting with his victory at the 2014 Bridgestone Invitational and ending when Jordan Spieth passed him with a runner-up finish in the 2015 PGA Championship.

Just don't get the idea Day is consumed with the ranking.

''I said earlier that a calendar year would be great to go No. 1, but I need to just focus on what I need to do because you can't really focus on staying No. 1,'' Day said. ''The more you focus on the actual target itself, the more you attach yourself to it, you make mental errors out there, you get more frustrated, you do silly things on the golf course that you shouldn't be doing.''

Johnson also had a mathematical chance to get to No. 1 last week, but he would have had to win Pebble and have Day finish out of the top 50. Day tied for fifth. Johnson also had a chance at the PGA Championship last summer, but he missed the cut and Day was runner-up.

Day doesn't have a lot of history at Riviera. The only reason he is playing is because of the tour's new ''strength of field'' regulation that requires most players to compete at an event they haven't been to in four years.

His best finish is a tie for 62nd.

He keeps reminding himself that he didn't have a great history at Bay Hill and The Players Championship, and he won them both last year.