SAN FRANCISCO -- It all felt pretty strange for Blake Snell.

No spring training with his new team to work out the kinks. Finally facing major league hitters for the first time in April. He couldn't wait to get back out there.

But Lane Thomas drove in three runs, and the Washington Nationals spoiled Snell's San Francisco debut, beating the Giants 8-1 on Monday night in the opener of the Nats' weeklong stay in the Bay Area.

"Honestly, it was just weird. I'm facing big league hitters, I was aware of that," Snell said. "I was being more careful than I usually would. Usually, I would just throw it over the plate.

"Then I was really amped up too. I was really excited to pitch. Bad combination I guess."

Snell, a two-time Cy Young Award winner who claimed the National League honor last season with the San Diego Padres, agreed to a two-year, $62 million contract with San Francisco on March 18. He had been throwing at extended spring training in Scottsdale, Arizona, to build up his arm strength.

Still, he likes how the ball is leaving his hand and looks forward to facing another former team, the Tampa Bay Rays, on Sunday in Florida.

"There's a lot going through my head right now, but I would just say I was pretty amped up, and I was trying to nitpick the zone too much," Snell said. "I made the bullpen have to cover a lot of innings that they shouldn't have had to cover."

Staked to a 1-0 lead on LaMonte Wade Jr.'s RBI double in the first, Snell (0-1) issued consecutive one-out walks in the second. Trey Lipscomb and Luis Garcia Jr. then hit back-to-back RBI singles, and Lipscomb scored as part of a double steal.

Snell was on a pitch count and finished at 72. He gave way to Erik Miller after the third, having allowed three runs and three hits with five strikeouts and a pair of walks.

The left-handed Snell received a warm ovation when he emerged from the dugout and walked out to the center-field bullpen to begin his warmup throws.

"When we signed him, it was a big deal," said San Francisco manager Bob Melvin, who also was Snell's skipper last year with the Padres. "It really kind of energized our clubhouse at spring training. Now the buildup, not ideal, and it's not perfect, but it is what it is. But I think in general, yes, everybody's excited about having him."

Snell had been 2-0 with a 2.33 ERA and 36 strikeouts over five previous starts against the Nationals, who follow this three-game set with a weekend interleague series across the bay versus the Oakland Athletics.

"At least he's off and running," Melvin said of Snell. "Not a great game for us all the way around."

The Associated Press contributed to this report.