Scottie Scheffler's streak of 21 sub-70 scores is over after he finished the opening round of the Procore Championship seven shots behind leader Mackenzie Hughes.

US Ryder Cup rookie Ben Griffin carded a six-under-par 64 and trails Canada's Hughes by one shot in Napa, California.

World number one Scheffler, who had tied with Patrick Cantlay for the longest streak of sub-70 runs, hasn't finished lower than eighth place since March.

"I think it was a pretty frustrating day overall," Scheffler said. "I felt like I did some things well out there, I just wasn't quite getting the reward."

Scheffler is set to make his third appearance at the Ryder Cup when it gets under way in America on 26 September and a nine more of the 12-strong US team are also using this week's event to warm up.

Russell Henley shot 65 and US Open champion JJ Spaun signed for 67 with the rest of the American Ryder Cup contingent all failing to break 70.

"I think it was a little different today just knowing that all of us were kind of paired in the same little area of tee times and we obviously did spend a lot of time with each other this week so it definitely feels slightly different than it would have if this tournament was in March," Henley said.

"You know, the Ryder Cup is right around the corner."

The top nine players are separated by just four shots with amateur Jackson Koivun among those in contention.

Leader Hughes, 34, is a two-time winner on the PGA Tour with the last of those triumphs coming in 2022.

After picking up nine birdies from the first 13 holes, bogeys on 14 and 15 upset hopes of a potential 59.