The three-times major champion notched five birdies to move five-under-par after 12 holes at Riviera, while South Korean Sung Kang was on the same score through 14 holes.
Only half the field made it to the tee due to a seven-hour rain delay.
Tiger Woods was among those left cooling their heels until Friday.
Play began on time but was halted less than an hour later at the storied course in the Pacific Palisades coastal neighbourhood of Los Angeles.
Officials took the unusual decision to scrap the scores and start the event again, citing poor visibility that had hampered the 30 players who had teed off.
"It never got where you could see where your ball went, barely see the flagstick," said rules official Mark Russell.
"That's not a common thing (to nullify scores), but the committee does have the discretion to do that. We felt in this situation that's the thing to do."
The last time a round was reset on the PGA Tour was at the 2013 Dell Technologies Championship.
Officials still hope to crown a champion on Sunday, but that will probably only be possible if there are no more delays.
More rain is forecast on Friday, before clearing for the weekend.
(Writing by Andrew Both and Frank Pingue; editing by Ken Ferris and Sudipto Ganguly)
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