Northern Irishman McIlroy rose to the rankings' summit for the first time since September 2015, taking over from American Brooks Koepka.

"I didn't know I was going to get a chance this early, but this has been a goal of mine for a while," McIlroy, 30, said.

"We all have egos out here and it would be great to get back there."

Former World No.1 Day moved up eight spots into the top 40 – from No.46 to 38, while compatriot Adam Scott led the Australian charge after rising one position to No.14.

“This has been a goal of mine for a while … We all have egos out here.” – Rory McIlroy

Day's outright fourth at Pebble Beach was his best result since winning the Wells Fargo Championship in May 2018.

"It's bittersweet, because I wanted to (win) but it's still a good step in the right direction," Day told AAP at the tournament's conclusion.

"You can find negatives in anything, but it's been a while since I've had a top five and it's a great result to have early in the year."

The Australian contingent in the top 50, sandwiched between Scott and Day, includes Marc Leishman (19) and Cameron Smith (35).