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.”