Rahm has won three PGA Tour titles
Spaniard Jon Rahm shot a seven-under 65 to take the lead in the clubhouse at the Farmers Insurance Open at Torrey Pines.
The 25-year-old world number three with holes for an eagle from 111 meters in the second and finished at 12 below.
Rory McIlroy from Northern Ireland, who needed a win to return to world number one, is nine down after a fine 67, with Tiger Woods two further back.
Ryan Palmer, two nights ahead after a 62 at Noordbaan, was 10 under with a handful of holes to finish.
The last two rounds are played on the more challenging Zuidbaan, but round three was delayed by two hours on Saturday due to dense sea fog along the California coastline.
Rahm quickly made up for lost time and played the first nine out of five and added another birdie on the 13th after playing the bunker to a few centimeters from the cup.
A sparkling tee shot up to three feet on the 16th gave him a fifth birdie.
McIlroy started the six shots around the lead but put together six birdies with some typically spectacular shots, to match his opening day score.
He pitched at the 607-yard 13th with his second shot, although three putts from there only gave him a par.
However, his birdie-putt on the 470-yard 15th was a lot easier while his beautiful approach from 155 meters ended one foot off the pin, while two majestic hits at the end of par five gave him another take a shot.
Woods, winner of eight titles at Torrey Pines, made an excellent start, playing the first nine in four under, including a chip-in from the green to the fourth.
He set three times to get the 11th bogey, but then ran from 15 feet to the last and make sure that only one was shot in his round of 69.
"It was important because I felt like I'd be left behind if I dropped another shot, "said world number six, trying to move. ahead of Sam Snead as the winner of the PGA Tour without a doubt.
"It was a positive move today, I got out early starting early, but I still have low places tomorrow – many boys are ahead of me."