Billy Horschel leads the way after the opening day of the Farmers Insurance Open in San Diego with world number one Jon Rahm the best of the big names in the field.

Horschel leads by one stroke from Michael Thompson after the 35-year-old American carded a bogey-free nine-under-par 63 on the North Course at Torrey Pines.

World number 23 Thompson also went bogey free on Thursday, birdieing four of his first five holes to jump out of the blocks, before birdies on the 17th and 18th holes rounded out a strong day.

Stephan Jaeger and Kevin Tway are seven-under-par, with Rahm on six-under-par alongside three others players; Rickie Fowler, Doug Ghim and Francesco Molinari.

Rahm's round on the South Course included two bogeys and six birdies but he jumped up the leaderboard with an eagle on the 18th.

"Oh, be good."

It was.@JonRahmPGA makes the only eagle of the day on 18 at the South Course. pic.twitter.com/CmCVDsCdhT
— PGA TOUR (@PGATOUR) January 27, 2022

Sam Burns, Daniel Berger and Tony Finau are among a group of players tied at five-under while Justin Thomas started with a four-under-par 64 but had five birdies on his front nine before two bogeys in his final five holes.

Tokyo Olympics gold medallist Xander Schauffele and 2020 Masters champion Dustin Johnson are also four under.

A bleach-blonde Brooks Koepka had two bogeys and four birdies to be two under, alongside three-time major winner Jordan Spieth and 2020 US Open champion Bryson DeChambeau.

New-look @BKoepka. pic.twitter.com/k5hbUPj2QF
— PGA TOUR (@PGATOUR) January 26, 2022