Nationals' starter Doug Fister's struggle to find consistent sink continues...

Doug Fister got off to a good start in Monday night's series opener with Arizona, but the Diamondbacks' hitters took advantage of some mistakes by the Washington Nationals' sinker-balling right-hander, who once again struggled to keep the ball down in the zone.

D-Backs' shortstop Nick Ahmed hit a belt-high 3-1 sinker out to left on a line in the third, putting the visitors up 1-0 early.

In the fourth, it was a first-pitch changeup to David Peralta that ended up clearing the wall in front of the Red Porch seats, just out of reach of a leaping Michael Taylor for a two-run blast that made it 3-0.

Wellington Castillo got an 0-1 sinker to hit in the at bat that followed and sent it out to the Red Porch seats in left-center field to put the Diamondbacks up 4-0.

Arizona added a run on a sac fly in the fifth for the final run of five they put up on Fister, who took his fourth loss in his last six starts.

Fister vs Ahmed:

Fister told reporters after the outing that the issue was obvious.

"Being up in the zone definitely," he said. "I'm going after them. I'm attacking with my best stuff and when it's up in the zone, it's much easier to hit. It doesn't have the deception. It doesn't have the sink on it. It's much flatter and straighter.

"And these guys are good hitters and I have to respect that and I have make a quality pitch and if not they're going to make me pay for it."

"All their runs came via the homer," Nats' skipper Matt Williams said, "which is an indication of the ball being up in the zone.

"A couple of balls hit the other way for homers tonight, ball was carrying, but still they're up."

Making the times that he struggles to keep the ball down more frustrating for him is the fact that there are stretches during which he's able to keep the ball down consistently, but it's the consistency that's the issue.

"For me the obvious answer is being able to go out there and repeat my delivery and repeat the quality of the pitch," the Nationals' 31-year-old right-hander explained.

Fister vs Peralta:

"Early on, again, I was able to make those pitches count and down in the zone, in off the plate, and I was doing that towards the end of the game too, so now it's just a matter of being more consistent."

"For him it's about down," Williams said. "The down angle, the ball moving down, and when the ball is up it doesn't have a chance to do that, so it moves laterally, especially to the left-handers and they can stay on the baseball and hit it to the middle of the diamond.

"If it's sinking then you find what you found in the first couple innings where it's rolling over, grounders, it's early count outs and he's in command. But when he gets up, it's a lot easier for a hitter to hit."

Fister's continued to work on finding the consistent delivery and mechanics that will enable him to keep the ball down pitch after pitch, but 15 starts in, with some time off for a DL stint, he's still searching.

"I think continuing to work through the trials that I've got," he said, "but just not executing every one of them that's my biggest downfall right now is I've got to be more consistent. I started out okay and ended okay, but really got to stay with it there in the middle."

How long can a team with postseason aspirations wait for Fister to figure it out?

Fister vs Castillo:

• We talked about Fister's struggles, the Nationals' fourth loss, Stephen Strasburg's rehab outing and more on the latest edition of Nats Nightly: