SAN FRANCISCO • World No. 1 Jon Rahm's Ryder Cup prep week has not gone quite according to plan after a stomach ailment, but he said he was improving following an even-par 72 in the first round of the Fortinet Championship, the first event of the 2021-22 season.

Rahm withdrew from Wednesday's pro-am with the bug, which he blamed on fatigue after battling to a runner-up finish behind Patrick Cantlay in the season-ending Tour Championship last week.

"I didn't have the mental clarity I usually have," he said, attributing that to the fact that he did not eat on the course as he usually does.

"It was a bit of a battle in that sense, that's why probably on the greens it wasn't my best day, but again, it is what it is. I've dealt with worse. I think a lot of people out there have played with worse.

"Tiger Woods won a US Open with a broken knee, so I'm OK."

Rahm, who said he felt "way worse" than when he was diagnosed with Covid-19 this year, had two bogeys and two birdies at Silverado Resort in Napa, California.

American Chez Reavie was the first-round leader with a seven-under 65.

His patience paid off as he bounced back from an early bogey at the 14th hole with birdies at the 16th and 17th, and he came alive on his inward run, his six birdies including three in a row to end his round.

"I told myself to stay patient, you have plenty of chances to make birdies, and I was able to do that coming in," said Reavie, who rolled in a 17-footer at the seventh, and drained a seven-foot birdie at the eighth before firing out of a greenside bunker to three feet at the ninth and making that for birdie.

"I was one-over through six holes, I think. I was like, OK, stay patient, don't become impatient. Fortunately I was able to make some birdies."

American Cameron Tringale and Canadian Adam Hadwin shared second on six-under 66 with six players sharing fourth on 67.

Most of the world's top golfers are absent this week, taking a rest after the Tour Championship or getting ready for next week's Ryder Cup duel between the United States and Europe at Whistling Straits in Wisconsin.

But Rahm is not taking a break as he prefers playing competition rounds and is preparing himself for the "mental aspect" of the biennial tournament, which was postponed by a year due to the Covid-19 pandemic.

In addition, the reigning US Open champion thought the set-up at Silverado would be better preparation for what he will face at Whistling Straits than he would find at golf courses in Arizona, where he is based.

"Right now in Arizona, it's hot. They're watering the courses a lot to keep the Bermuda (grass) in proper shape for the season, so everything's a little bit slower, a little bit longer, so it plays nothing like Whistling Straits would," he said.

"That's one of the reasons why I wanted to come here because at least I'm playing a championship golf course, it's firm, it's fast."

Rahm also admitted he was fired up after coming up short this week in voting for the PGA Tour's Player of the Year honours, which went to Cantlay despite "playing amazing golf".

AGENCE FRANCE-PRESSE