The billing couldn’t have been bigger. Dodgers vs. Giants. Yoshinobu Yamamoto vs. Logan Webb. One of the game’s oldest rivalries, pitting what were supposed to be two of the game’s top pitchers.

On Friday night at Dodger Stadium, however, only one right-handed ace showed up.

Webb did his thing, giving up two runs over seven spectacular innings.

Opposite him, Yamamoto was no match, floundering in a five-run, 4⅔-inning start in the Dodgers’ 6-2 defeat —one that left the rivals tied atop the National League West with identical 41-29 records after their first meeting of the season.

The evening was a study in pitching excellence (or, in Yamamoto’s case, a lack thereof); serving as a reminder that, for as good as Yamamoto has become in his second major league season, there are tiers to his talent he has still yet to reach.

“There were absolutely no pitches with which I was satisfied,” Yamamoto said in Japanese.

“I think the stuff is good,” added manager Dave Roberts. “I think he was kind of just being too fine.”

Indeed, where Webb got soft contact and quick outs, needing just 98 pitches to complete his seventh seven-inning outing of the season, Yamamoto labored through hitters’ counts and long at-bats, issuing a career-high five walks while finding the strike zone on just 56 of his 102 pitches.

Where Webb limited traffic and escaped rare trouble, giving up only two hits while walking three batters, Yamamoto toiled through self-inflicted jams; none worse than when he walked the bases loaded in the third, then gave up a tie-breaking grand slam to Casey Schmitt.

San Francisco’s Casey Schmitt, right, celebrates with Wilmer Flores, center, and Mike Yastrzemski after hitting a grand slam in the third inning Friday.

(Gina Ferazzi / Los Angeles Times)

“The way I gave up runs was really bad,” said Yamamoto, who ripped his glove off in disgust, and almost chucked it to the ground, as Schmitt’s drive disappeared beyond the left-field fence.

“I tried to regain my rhythm and pitch better. I tried to turn the page emotionally. But I wasn’t able to adjust. I didn’t pitch well until the end.”

Yamamoto did indeed settle down after the grand slam, allowing no other runs while getting into the fifth inning.

But, where Webb played the part of a contending team’s staff ace, lowering his earned-run average to 2.58 (fifth-best in the National League), Yamamoto faltered in a way that’s become uncomfortably familiar of late, his minuscule ERA having almost tripled over the last month.

In his first seven starts, Yamamoto was 4-2 with a 0.90 ERA, a 0.925 WHIP and had only one game in which he gave up even two earned runs.

“Right now, he’s pitching like the best pitcher in the world,” catcher Will Smith said on May 2, after Yamamoto spun six shutout innings against the Atlanta Braves.

But since then, Yamamoto has been on an entirely different, much less reliable planet.

Over his last seven outings, the 26-year-old Japanese star is 2-3 with a 4.46 ERA. In that span, he has more starts of less than five innings (two) than of seven full innings (one). He has given up three or more runs four times, and a career-high-matching five runs twice. And with his season ERA at 2.64, he’s trending in the wrong direction.

“A game like this, I just need to focus, learn things, and turn something into a positive,” Yamamoto said. “And then get myself ready for the next outing.”

The most consistent problem during Yamamoto’s slump: Poor command.

Even with a tight strike zone from home plate umpire Adam Beck on Friday, Yamamoto blamed his own poor execution as the primary factor. He has walked 17 batters in his last 38 ⅓ innings. His usual pinpoint control has suddenly disappeared.

Dodgers pitcher Yoshinobu Yamamoto delivers against the Giants on Friday night.

(Gina Ferazzi / Los Angeles Times)

It’s a problem Roberts believes explains Yamamoto’s overall downturn. And when it hasn’t led to free passes, it has put the pitcher behind in hitter-friendly counts — like when Willy Adames opened the scoring Friday by getting ahead 2-and-0 and hitting a down-the-middle fastball to right for a solo home run.

“I think that’s correctable in the sense of, I think it’s just an intent part of it,” Roberts said. “It’s not like the stuff was backing up [on him].”

Another potential factor in Yamamoto’s struggles: He has recently been forced to pitch on less rest between starts.

Over his first seven starts, Yamamoto pitched on at least six days of rest — mirroring the once-per-week schedule he had in Japan.

Since then, however, each of his outings have come on shorter five-day breaks.

Yamamoto has repeatedly downplayed that factor, saying he hasn’t noticed any physical diminishment on his new schedule. Roberts noted how last year, Yamamoto actually had slightly better numbers on five days of rest (2.97 ERA in 11 starts) than six (3.07 ERA in seven starts).

Dodgers catcher Will Smith scores past Giants catcher Andrew Knizner during the second inning Friday.

(Gina Ferazzi / Los Angeles Times)

Still, as the Dodgers have navigated around their shorthanded pitching staff, Yamamoto’s drop-off has come at a particularly bad moment. Not only have the Dodgers been in the midst of a grueling portion of their schedule over the last month, but they have watched the three-game division lead they held at the end of May evaporate just 13 days into June.

“It’s really critical,” Roberts said of this current homestand, in particular, with the third-place San Diego Padres due in town for a four-game set next week after the Giants leave. “You see the schedule, you see the opponents, you see where your ball club is at, and you just want to keep trudging along and play good baseball.”

The Dodgers’ lineup, of course, didn’t help on that front, either, Friday.

After scoring on an Andy Pages sacrifice fly in the second, when a throw home beat Smith but was dropped by Giants catcher Andrew Knizner while trying to apply a tag, the team’s only other production against Webb came via Teoscar Hernández, who lined the Dodgers’ first hit to right field in the fourth before homering for a second-straight game on a solo blast in the seventh.

By then, however, Webb had already put the game on ice, becoming the latest starting pitcher this month to handle the Dodgers’ star-studded lineup (opposing starters have a 2.43 ERA against the Dodgers in June, and are averaging almost six innings per start).

“I thought early, it was going to be a tight game, and we might have been able to get to him,” Roberts said. “But I think once they hit the grand slam, then it just gave him a lot more margin, right there, and he just was on the attack.”

Which, once again, made Yamamoto’s clunker all the more costly; magnifying the problems facing a talented pitcher that the team desperately needs to again start performing like an ace.

Said Roberts: “It just wasn’t as efficient as it needed to be tonight.”