At that point, Washington will have cleared two significant roadblocks in its pursuit of the NL East title.
Burnett scattered six singles over seven innings, Ryan Howard had two RBIs and the last-place Phillies beat Washington 3-1 Saturday to secure their first winning season against the Nationals since 2010.
Dominic Brown homered for the Phillies, who have won five straight against the division leaders and hold a 10-8 advantage in the series that ends Sunday.
So what is it about the Phillies that proves so troublesome to Washington?
"Over the last five it hasn't been good for us," Nationals manager Matt Williams conceded. "It's been a case of them pitching really well against us, Burnett especially."
The 37-year-old Burnett (8-15) allowed one run, struck out four and walked two. He improved to 4-2 with a 3.57 ERA in six outings against Washington this year.
"He's a good matchup against our guys," Williams said. "He seems to throw strikes against us. His walk total against us is certainly lower than the other teams he faces. It's one of those things in baseball."
Burnett's 80 walks are most in the majors, but against Washington this season he's got 33 strikeouts and just 15 walks.
"He just goes out there and pitches well," Nationals left fielder Bryce Harper said. "He locates. He mixes it up a lot. I got some pitches today I thought I could hit and missed. That's something you just got to tip your cap to. So be it."
It was Burnett's 400th career start. The right-hander lost his previous six road starts, and his only three wins since June 20 are all against Washington.
"I was able to make pitches at the right time," Burnett said. "Threw a lot of balls in there for strikes. Important that I stayed ahead in the count, especially against that lineup."
Tanner Roark (12-10) allowed three runs and six hits in six innings for Washington, striking out eight and walking one. He has lost his last three starts.
"Felt good, felt like I was commanding both sides of the plate," Roark said. "Everything was sharp."
On most nights, his numbers would translate to a victory. Not this time. Not against the Phillies.
"Just one of those things," Roark said. "We're still in first. We can't really think about that at all."
The Nationals finished with seven hits, all of them singles, ending their streak of 13 straight games with a home run.
Before the game, Williams said he would take a closer-by-committee approach while struggling Rafael Soriano attempts to correct some mechanical flaws that have attributed to his 6.98 ERA since the All-Star break. Williams offered no guarantee that Soriano would return to the closer's role.
Burnett hit the first of four Philadelphia singles in the third inning. The last, by Howard, provided the Phillies with a 2-0 lead and increased the slugger's team-leading RBI total to 91.
A sacrifice fly by Anthony Rendon made it 2-1 in the fifth, but Philadelphia got the run back in the sixth when Brown drove a 3-0 pitch into the Nationals bullpen beyond the right-field wall.
Roark faced one more batter before being pulled.
"It all started with a base hit by the pitcher," Williams said. "He threw Howard a curveball he hit into right field. And the 3-0 homer. Other than that he was pretty good."
Their current records don't show it, but the starting pitchers in Sunday's series finale -- Washington's Gio Gonzalez (7-9) and Philadelphia's Cole Hamel (8-6) -- rank third and fourth among NL lefties in wins over the last three seasons. Gonzalez has 39 and Hamels has 33.