San Francisco - Phil Mickelson says he's pleased with his opening round of the 2018-19 PGA season, but warned people not to read too much into his seven-under 65 at the Safeway Open on Thursday.

"I was just surprised at some of the shots I hit," the left-handed American said. "Today was an anomaly. Don't let the good round fool you. I am not at my best."

Mickelson came into the tournament at the Silverado Resort and Spa fresh off a disappointing appearance at the Ryder Cup where he went 0-2 as the Americans suffered a devastating defeat to Team Europe.

"I am tired and fatigued," he admitted. "I am trying to save every ounce of energy for the course."

He took advantage of the birdie-friendly Napa, California, course to sit in third place, two strokes back of the leader Sepp Straka.

Mickelson rolled in six-straight birdies beginning at the par-five No. 9 and said he had no inkling when he got up in the morning he would play so well.

"I had one of my worst warmups of the year," he said. "But you throw on one of these shirts and you can dance and hit fairways, awesome."

Austria's Straka fired a nine-under 63 while American Chris Wright is alone in second after shooting a 64.

Brandt Snedeker and Mackenzie Hughes were part of a group of eight players who shot six-under 66. Over 100 players in the 143-man field finished under par on Thursday.

Most of the game's biggest names are taking time off after the rigors of the FedEx Cup playoffs and won't return until January.

This opens the door for a collection of Web.com Tour graduates to try and cement their status on the main tour.

Mickelson said he felt like he "wasted his time" trying to battle the narrow fairways and deep rough at the Ryder Cup in France, where Team Europe triumphed 17.5-10.5.

"They had brutal rough, it was almost unplayable and it is not the way I play," Mickelson said. "Here I can miss some fairways and it is playable.

"I am 48-years old, and I am not going to play tournaments with rough like that anymore. It is a waste of my time.

"I am going to play courses that are playable and I can play aggressive, attacking, and birdie-style golf."

There were reports of some infighting among the Americans on the Ryder Cup team in France, but Mickelson said he didn't notice it.

"We had one of the best weeks as far as teammates working together and camaraderie.

"I didn't see any of that stuff happen. It was one of best weeks for team unity in long time."

Scores from the first round of the Safeway Open in Napa, California on Thursday (USA unless stated):

63 - Sepp Straka (AUT)

64 - Chase Wright

65 - Phil Mickelson

66 - Alex Prugh, J.T. Poston, Mackenzie Hughes (CAN), Brandt Snedeker, Sam Saunders, Im Sung-Jae (KOR), Adam Long, Wyndham Clark

67 - Martin Laird (SCO), Jonathan Byrd, Richy Werenski, Dylan Frittelli (RSA), Roger Sloan (CAN), Ryan Moore, Brendan Steele, Troy Merritt, Kyle Jones, Adam Schenk