A bogey on the last has stymied the progress of Jason Day, who is four shots off the lead after steadily climbing up the leaderboard at the PGA Tour's Farmers Insurance Open in San Diego.
The former world No.1 shot a 4-under 68 at Torrey Pines' North Course after making three birdies in four holes to edge his way towards the top.
But a dropped shot on the par-five ninth - his last hole - left him in a tie for 19th, behind a congested leaderboard.
The Australian began his round with an opening birdie before posting bogeys on 14 and 15.
Day bounced back on the 16th and caught fire coming home with further birdies on the 1st, 4th, 5th and 7th, before his disappointing closing hole.
The field is chasing Sam Ryder, Brent Grant and England's Aaron Rai - three players who have never won a PGA Tour event in a combined 203 starts.
All three shot 8-under-par 64s at, leaving them one stroke ahead of Brendan Steele. None of the trio ever previously led or co-led after a round on Tour.
Sahith Theegala, Hayden Buckley, Sam Stevens, Andrew Novak, Gary Woodland, Harry Higgs and South Africa's Garrick Higgo are tied for fifth at 66. All of them played the North Course except for Stevens and Novak, who played Torrey Pines' South Course.
The players will play one round apiece at the two courses on Wednesday and Thursday before the final two rounds are contested strictly at the South Course on Friday and Saturday.
Ryder, 33, has played on the PGA Tour 146 times previously without posting a win. He got his bogey-free round started in fine fashion when he eagled the par-5 10th hole. Ryder added three birdies on the front nine and three more on the back nine.
"Great to start off with an eagle," Ryder said. "Scorable conditions, drove it great, ironed it well, rolled a couple putts in."
Rai, 27, is a two-time winner on the European Tour who has played 44 times previously on the PGA Tour. His bogey-free day began with a birdie at No.10, and he closed each nine with back-to-back birdies.
"Really drove it great off the tee, which is probably the most important thing to at least give ourselves an opportunity with those approach shots," Rai said.
Grant, 26, is a PGA Tour rookie who made the cut in just two of 13 prior starts. Starting on the front nine, he birdied four of his first five holes, then bogeyed the par-5 sixth hole. Grant added four consecutive birdies on either side on the turn and added one last birdie at No.17.
"I just made some putts and hopefully we keep doing that and scores will show," Grant said.
Spain's Jon Rahm, who is attempting to become the first player to win three consecutive starts on the PGA Tour since Dustin Johnson in 2016-17, shot a 1-over 73 on the South Course and is tied for 116th in the 156-player field.
His streak of making 21 consecutive PGA Tour cuts, the longest active run on the circuit, could be in jeopardy.