Rory McIlroy was denied another "euphoric moment" in becoming world number one for the first time since September 2015 and admits he faces a tough task to stay at the summit.

McIlroy and Brooks Koepka did not contest last week's AT&T Pebble Beach Pro-Am, but McIlroy moved above the American due to the vagaries of the two-year rolling ranking system.

Both are in action at this week's Genesis Invitational, where former US Open champion Webb Simpson is the only member of the world's top 10 not competing in a star-studded event hosted by Tiger Woods at Riviera.

This is McIlroy's 96th week in total as world number one, one behind the record for a European player held by Sir Nick Faldo, although his lead over fellow four-time major winner Koepka is just 0.03 points.

"Everyone keeps saying congratulations but the work's only started, it's staying there is the hard part," McIlroy told a pre-tournament press conference.

"It's a calculation based on how you've played the last two years and I've played consistently well and the mathematics add up that I'm top of the list right now and I have a chance to stay there this week.

"I've always said it's a by-product of doing the right things week in, week out, playing well, trying to win tournaments and if I can continue to do that then hopefully that thing takes care of itself."

Asked if it was something of an anti-climax to become number one during a week off, McIlroy added: "I've already that euphoric moment in 2012; I won the Honda Classic to get to number one in the world, Tiger was coming down the stretch, I was able to hold him off. That to me was the really cool moment.

"At least I've had one of them, that's nice. To me it doesn't matter if I was sitting at home on the couch and got there or whatever, it is what it is.

"So I've already had that experience and I think as long as you do get that experience, especially the first time you get there, that's all you need."