Garnett won in 2018 and has come back here each year. He prefers a steady wind and feels comfortable on the greens, and it showed.
Martin matched Garnett with a bogey-free round, joining him in the lead with a birdie on the par-3 eighth and finishing with a tee shot off the rocks on the par-3 ninth that took a favourable bounce and landed onto the green.
They had a one-shot lead over Englishman Matt Wallace.
The tournament is held opposite the WGC-Dell Match Play in Texas. The winner does not get an invitation to the Masters, and no one in the field can reach the top 50 in the world, the other avenue to Augusta National.
Garnett began his round with a 20-foot birdie putt on No.10, but the key was the par 5s. He made birdie on all four of them, part of his plan on the Corales course at Punta Cana.
"If I can get the par-5 birdies, I'll usually have a pretty good day, and I was able to get all four today," Garnett said. "That was probably the biggest difference shooting 2 under or 6 under."
Martin did it the hard way. He made only one birdie on a par 5, but his game was solid the rest of the day and he converted the chances he had. And then there was the matter of that big break at the end of his round.
"Hit a very questionable shot that hit the rocks and somehow ended up on the green," Martin said.
"It's nice to get good breaks. We probably get a few more bad bounces than we do good throughout the season, so took advantage of that, stayed bogey-free."
The tournament is a good chance for players to move up in the FedEx Cup in a season where only the top 70 reach the post-season. Of the leading nine players, Martin (No.89) is the only one among the top 100.
The group at 68 included Dylan Wu and Sean O'Hair.
Roughly half of the 120-man field were at par or better, mostly a product of the wind that blew all day in the Dominican Republic.
Greg Chalmers was one of those who managed par and was the leading Australian.
Cameron Percy and Aaron Baddeley both had 74, while Harrison Endycott and Geoff Ogilvy are a shot further back.
