Day's No.1 golf ranking under siege in LA

Jason Day's golf world No.1 ranking is under siege as American Dustin Johnson sits atop the leaderboard at the US PGA Tour's Genesis Open.

Big-hitting Johnson carded a second consecutive five-under-par 66 to take a one-shot halfway lead from Pat Perez and Cameron Tringale on Saturday.

He was unable to tee off in the delayed third round before darkness halted play at the rain-hit Los Angeles tournament but he has plenty to play for on a marathon Sunday.

World No.3 Johnson can usurp Day with a victory if the Australian finishes worse than a three-way tie for third.

That is a strong possibility with the Australian nine shots back at one-under and in a tie for 53rd after five holes in his third round at Riviera Country Club.

Last year's US Open champion Johnson was unfazed at the prospect of ending Day's reign of 47 consecutive weeks as world No.1.

"I would like to be the best in the world but you get there winning golf tournaments," said Johnson.

"If I win this week and I happen to get to No. 1, great. I'll definitely be excited (and) proud."

After two consecutive 70s, 29-year-old Day did himself no favours with bogey on the reachable par-5 11th early in his third round.

World No.7 Adam Scott (one hole), Cameron Smith (four holes) and Cameron Percy (first hole) finished the interrupted third round as the leading Australians, tied for 17th and five shots behind Johnson.

Scott - the 2005 champion at Riviera - is confident he can still make a run at the title and do Day a favour over a marathon Sunday.

"Dustin is the guy to catch right now, but five back isn't too bad when there's not too many people in front," said Scott.

After starting the second round at four under, Marc Leishman carded a five-over 76 to join Greg Chalmers (one over) in finishing a shot outside the even-par cut.

Other Aussies who missed the cut were John Senden (two over), Aaron Baddeley (three over), Geoff Ogilvy and Steven Bowditch (four over).