WASHINGTON -- The Giants found themselves without their usual leadoff hitter after Mike Yastrzemski tested positive for COVID-19 on Sunday morning. They filled the void by turning to Joc Pederson, who didn’t miss a beat after slotting into the top of the order for the first time this year.

Pederson went 3-for-3 with two home runs, a double and three RBIs to help the Giants complete a three-game sweep of the Nationals with a 12-3 rout in Sunday’s series finale at Nationals Park.

Pederson, 30, quickly set the tone for San Francisco, crushing his 24th career leadoff home run on Nationals right-hander Joan Adon’s fourth pitch of the game. Pederson’s towering shot traveled a projected 441 feet out to right field, making it the longest leadoff homer by a Giant since Statcast’s inception in 2015.

The Bay Area native extended the Giants’ lead to 3-1 with a sacrifice fly in the second inning and then homered again in the seventh, when he smoked another no-doubter off Washington reliever Austin Voth to make it 6-3. It was the 18th career multi-homer game for Pederson, who is tied for third in the Majors with five home runs over 13 games this year.

“I think he set the tone for the day, obviously, in his first at-bat,” manager Gabe Kapler said. “But I think he’s really picking good pitches to go after. I think even when he gets deeper into counts and pitchers have velocities, he’s able to catch up to that velocity. It’s a good sign. His bat speed is there. His rhythm is there. I think he’s getting more and more confident.”

Signed to a one-year, $6 million deal in March, the left-handed-hitting Pederson has emerged as one of the Giants’ top hitters so far this year, leading the club’s regulars in home runs (5), batting average (.364) and OPS (1.133).

“He’s awesome,” right-hander Logan Webb said. “He’s always been fun to watch. My first two years, he was with the Dodgers, so I loved watching him, but obviously I wanted us to get him out and stuff. It’s good to see him on our side now. He brings that energy every day. He’s got the blond mohawk. It’s cool to see, and it’s fun to be a part of.”

Despite Pederson’s monster day at the plate, the Giants remained committed to their platoon system, with Austin Slater coming in to pinch-hit for him against Nationals lefty Sam Clay in the ninth. The Giants went on to break the game open with a six-spot, which was capped by Slater’s two-run single in his second at-bat of the inning.

“It just shows the depth and the versatility of the lineup,” Pederson said. “Just because you don’t start the game doesn’t mean you can’t come into the game later and help contribute. I think you see that a lot here. There’s so much talent in this room. No one person thinks they need to take care of the whole job themselves. There’s a lot of trust in the person behind them. … I think that’s really smart. It’s a common goal of wanting to do whatever we can to win a World Series, and that’s sacrificing individual statistics.”

Kapler described Pederson as an “A++ teammate” who fits right into the unselfish culture the Giants successfully cultivated last year, when they stunned the industry by winning a franchise-record 107 games.

“We have guys who kind of understand how this roster is constructed,” Kapler said. “We have guys who understand the strategy that we’re rolling out there. Joc has just been on board from the very beginning. It really doesn’t mean that he’s not going to face lefties in big situations. His true talent level is that he’s capable of being a 30-home run, 100-RBI, 20-plus steal, regular player. It just might be that he’s even better with some blows against some of these nastier lefties around the league. Not to say he can’t hit those guys, but he just might be better.”

The Giants’ bats, which cranked out 15 hits on Sunday and outscored the Nationals 24-6 this weekend, provided plenty of backing for Webb, who rebounded from a rough start against the Mets on Tuesday by delivering a quality start in his fourth outing of the year. Webb gave up three runs on seven hits over 6 2/3 innings to improve to 2-1 with a 2.96 ERA on the season.

All three of the Nationals’ runs were driven in by Yadiel Hernandez, who briefly cut the Giants’ lead to 4-3 with a two-run shot off Webb in the third. Hernandez’s blast, which sailed into San Francisco’s bullpen in left field, marked the first home run allowed by Webb in 60 innings, a streak that dated to Sept. 12 and included his two playoff starts last fall.

“I just feel like I’m finding myself a little bit right now,” Webb said. “I don’t feel like I’ve honestly got my best stuff right now. You go through stretches like that. [Pitching coach Andrew Bailey] always tells me that there are parts of the season where you don’t feel great. I feel good, but it’s just not there right now, so we’ll look into some mechanical things.”