The young San Francisco Giants, who were supposed to fear this trip into the lair of the seasoned Cicinnati Reds, opened a character-testing four-game series by shelling the Reds ace, Tom Seaver, and winning the opener of last night's doubleheader, 7-6.

Home runs by Willie McCovey (No. 503 of career) and Darrell Evans helped the first-place Giants to a 5-0 lead over Seaver and the Reds after five innings.

The Giants, who entered the contest with a three-game lead over the Reds and Los Angeles Dodgers in the Nationsl League West, survived a three-run uprising in the ninth to preserve the victory for starter Vida Blue (12-4).

Cincinnati, which came away from a bases-loaded non-out rally in the sixth without a run, opened the ninth with singles by Pete Rose and Ken Griffey. Rose, with three hits, ran his hitting steak to 22 games, longest in the NL this year.

After two ground outs, the second socoring a run, the Red's Johnny Bench - plagued with back miseries for six weeks - crashed his first home run since June 2 to cut a 7-3 lead to 7-6.

Ace reliever Gary Lavelle of the Giants got the Red's Dan Driessen to fly out deep to center to end the game.

A pitcher's duel, not an all-fields battle of line drives, was expected with a pitching matchup of Tom Seaver and Vida Blue, flame throwers hurling in the twilight game of this doubleheader.

Seaver never reached the sixth inning. Blue never got out of his sixth. It took Tom Terrific little time to learn that even the shadows cutting across in front of the mound were not enough to make him unhittable.

Willie McCovey took one of his long, classix swings at Seaver's first pitch of the second inning and crashed a towering fly into the second deck of seats in right for his 503 rd home run. McCovey stood at the plate, hands characteristically on top of head, and watched his 400-foot masterpiece alight.

Darrell Evans followed with another 400-foot fly. Red's centerfielder Cesar Geronimo camped under it non-chanlantly in front of the 404-foot sign. Geronimo made his one-handed flick of the glove and - "Geronimo" - the ball fell from his mitt onto the warning track for three bases worth of error. Vic Harris promptly scored Evans on a sacrifice fly to left. George Foster looked rather unemotional, running in and to his left for the trotting catch, then never attempting the modest 250-foot throw to home.

The Red's outfield had just begun its nonsupport. The Giants, entering the game with a three-game division lead scored three times in the fifth for a 5-0 lead.

With one out, the Gaints' Terry Whitfield, with 20 hits in his last 52 at bats, singled to center. The Giants other little-known 300-hiutting outifelder - Jack Clark, who has six homers and 13 RBI in his last eight games - singled off Dave Concepcion's glove into short left.

Whitfield, a .305-hitting whippet acquired from the Yankee chain, dashed to third. Foster, charging the Clark hit which had died to a dribble, took his eye off the ball, watching Whitfield, and it rolled past him.

As Foster nearly did a back-flip chasing his incredible error. Whitfield scored and Clark took second.

Scaver already was rattled and upset. Roger Metzger had earlier lined a single off his right leg (only a Pete Rose dive had saved it from richocheting into the Giant dugout for a double off the pitcher's shin).

Fosters escapade, on top Concepcion's earlier blunder, may have distracted Seaver. Certainly his first pitch to Evans, after McCovey grounded out, was the sort of hanging curve that hitters dream about.

Evans conked his eight homer, a two-run, barely over the 375-foot sign in right.

The Reds answered with a quick kayo of Blue. In the fifth Geronimo and Dave Collins (pinch hitting for Seaver) singled. Rose ripped an RBI single off the right field wall. Collins took third on the frozen rope and scored on a Ken Griffey sacrifice fly.

"Vida Blue pitches the way a man should," Red Manager Sparky Anderson said before the game. "It's like playing back home in the pasture." Vida says, "I'm gonna throw hard.' Can you guys hit baseball?"

"Now that's baseball," Anderson said with a chuckle.

Against Cincinnati, even this injured facsmile of the Reds, that's suicide.

Blue, who framed 11 in his previous outing against the Reds, had seven waiffs in his five fast ball-filled innings. But he also allowed eight hits and three walks.

After being greeted with a leadoff walk and single in the sixth. Blue was replaced by Charlie Williams, the hidden rightly in the S.F. bullpen.