The source of the frustration oozing from Astros starter Lance McCullers Jr. on Friday night was the fifth inning, when Padres hitters Manny Machado and Jake Cronenworth had consecutive two-out, two-strike RBI hits on his curveball that wound up being the difference.

The Padres took advantage of some key scoring chances against McCullers -- scoring three runs on two-out hits -- while the Astros (who racked up 23 runs in their previous two games at Colorado) struck out a season-high-tying 13 times and went 4-for-16 with runners in scoring position in a 4-3 loss to the Padres at Petco Park.

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“Physically, I’m good,” McCullers said. “I just think sometimes my stuff throughout the games or throughout batters is doing different stuff than I’m accustomed to. I think that’s just part of coming back from the surgery and getting the feel.”

The loss snapped the Astros’ season-high eight-game winning streak.

“Boy, that was a tough one to lose, because you look back at the baserunners we had out there, with some guys who are starting to swing the bat batter,” Astros manager Dusty Baker said. “It’s frustrating.”

The Astros trailed, 2-1, heading to the fifth when McCullers was one out away from stranding Trent Grisham at second following a leadoff double. McCullers had Machado in a 1-2 hole before giving up an RBI single on a curveball.

“I had the opportunity to get an out there and carry over the game into the sixth,” McCullers said. “Kind of my Achilles’ heel this year. I gave up [a lot] of two-strike hits. I don’t really know what to attribute it to. They’re on a variety of pitches as well. I’ve just got to make some better pitches on two strikes. It’s a learning curve; it’s a process.”

McCullers then had Cronenworth in an 0-2 hole before he laced a double into right field to score Machado and make it 4-1. McCullers came back to strike out Wil Myers, ending his outing with a season-high-tying 97 pitches. He generated only five swings-and-misses: three on his sinker and two on his curveball.

Before the Tommy John surgery, McCullers threw curveballs on 46.6 percent of his pitches in the 2018 season, but this year his sinker has become his primary pitch (39.3 percent). Hitters were slugging .618 against his curveball entering Friday, as opposed to .429 on his sinker.

“I threw some good ones, some not,” McCullers said of his curveball. “I got a double play on one and some swings-and-misses towards the end. … A lot of the two-strike ones are hanging up there. Not even hanging, but just in the zone when I’m trying to expand. I’ve been doing a lot of midweek work on a lot of my other pitches, so maybe I’ll spend some time on the curveball.”