In his first full season in the majors, Roark (4-4, 2.91 ERA) has allowed two runs or fewer in eight of 12 starts while holding opponents to a .221 average. The right-hander is among the NL leaders with 14.9 pitches per inning.
"I'm just doing my thing. I'm throwing strikes, not worried about what team we are facing," he told MLB's official website. "I'm not trying to make perfect pitches. I'm throwing it and letting it go."
Roark had posted a 2.25 ERA during a three-start losing streak while receiving a combined three runs of support before striking out a career-high 11 over eight innings of three-hit ball in Friday's 6-0 win at San Diego. First-pitch strikes were key for Roark, who was consistently getting ahead in the count.
"That really gets a hitter on his heels and gives you confidence," he said. "I don't try to strike guys out. I just try to make quality pitches. Strikeouts happen."
Roark threw 77 of his career-high 107 pitches for strikes.
"He's just been pitching like that for us all year," manager Matt Williams said. "He just continues to pound the zone, all of his pitches. Pretty impressive."
The Nationals (34-29) have won three straight and nine of 11 behind a 1.34 ERA, outscoring foes 64-19. Doug Fister threw seven scoreless innings in Tuesday's 2-1 victory as Washington improved to 14-7 against San Francisco since 2011.
"They've got a well-rounded team," losing pitcher Madison Bumgarner said of the Nationals. "It comes down to whether they execute or not, but they've definitely got the talent over there."
The Giants (42-23) are on the brink of a season worst-tying third consecutive defeat after going 1 for 10 with runners in scoring position. San Francisco, which dropped three straight April 17-19, has scored three runs in the last two games.
''We've had so many timely hits from everybody that you think it's going to happen again,'' Giants manager Bruce Bochy said. ''Came up short, didn't happen.''
Cain (1-3, 3.52) is set to make his second start since coming off the disabled list with a hamstring issue. He yielded two runs and three hits over seven innings but didn't receive a decision in Friday's 4-2 victory over the New York Mets.
The right-hander is 3-0 with a 1.66 ERA in five home starts against Washington dating to 2007. Ian Desmond is 5 for 12 with a homer and a double against Cain, but Ryan Zimmerman is 1 for his last 10 in their matchups.
Cain will be getting his first look at Anthony Rendon, who had two hits in his first game back since missing three with a hand injury. Rendon is hitting .429 with four homers and eight RBIs during a seven-game stretch.