Irish pair Seamus Power and Pádraig Harrington are in a battle to make the cut following slow starts at the Valero Texas Open.
Power sits on level par in San Antonio, nine shots off leader Sam Ryder, with Harrington, one of the early starters on Thursday, a further shot back.
Power, starting on the 10th, was over par after a bogey at the 11th, but birdies at the 14th and 17th left him one-under par after his opening nine holes.
A bogey at the second was followed by seven straight pars to leave him just inside the cut-line ahead of Friday's second round.
Harrington also started at the 10th and was two-under par halfway through his round, but he struggled coming in, carding four bogeys and a birdie.
Sam Ryder birdied six of his final seven holes to take the lead on nine-under par.
Ryder was tied for the lead with Keith Mitchell (64) when he stepped to the tee at the par-five 18th. He stopped his third shot about a foot from the hole and tapped in for the go-ahead birdie.
Mitchell is alone in second, while Brian Harman is third after a six-under 66. Jordan Spieth is tied with Carson Young at five-under 67.
Ryder, 35, has never won on the PGA Tour and holds the No. 154 ranking in the world. If he wins the tournament, he would earn an invitation to the Masters next week.
"I've been focused...I feel like my game's been in a good place and I've been generally healthy so I feel like I know kind of my DNA as a player and I've been able to just kind of hone in on that," Ryder said after posting his lowest-scoring round of the season.
"The putter's been good this year, I think that contributes to my consistency."
Other notable players who are tuning up for next week's major in Augusta include Englishman Tommy Fleetwood (68), Sam Burns and Tony Finau (69) and Patrick Cantlay, Keegan Bradley and Swede Ludvig Aberg (71).
Rickie Fowler (75), Max Homa (76), Englishman Matt Fitzpatrick (76) and South Korea's Tom Kim (77) are in danger of missing Friday's 36-hole cut.
Defending champion Akshay Bhatia opened with a 74.
Additional reporting: Reuters
