WASHINGTON -- Edwin Jackson began the season in the minor leagues, but Thursday night he found himself pitching for a possible playoff roster spot with the National League East champion Washington Nationals.

Still, that seemed hardly important after his baseball-fan grandmother died a few days ago.

"That is the last thing on my mind," Jackson said of a postseason spot after he threw six innings and allowed just two runs and four hits as the Nationals beat the Pittsburgh Pirates 5-4.

Alejandro De Aza delivered an RBI single with no outs in the bottom of the ninth inning, giving the Nationals their ninth walk-off win of the season. De Aza, like Jackson, spent part of this year with Triple-A Syracuse in the Washington system.

Jackson, who did not figure in the decision, said he had nothing to lose in his last start of regular-season play. He remains 10-3 in his career against the Pirates as he walked two and fanned seven, one shy of his season best.

"Leave it on the field. I've always known what I can do," he said. "I wish September could have been as good as August was."

Said Washington manager Dusty Baker: "He threw the ball well tonight."

Does Jackson have a shot at the postseason roster?

"We will have not discussed it yet," Baker said.

The hit by De Aza down the first base line off Daniel Hudson (2-7) scored Anthony Rendon, who led off the ninth with a single and took third on a single by Daniel Murphy.

Pittsburgh's Josh Bell hit his 26th homer of the year, a two-run shot off closer Sean Doolittle, to tie the game at 4 in the top of the ninth. It was the first blown save in 22 chances for Doolittle with the Nationals this year. Andrew McCutchen had led off with a single.

"Now he can start all over," Baker said of the Doolittle streak. "Better now than later."

Bell tied a franchise record for rookie homers in a season, set by Jason Bay in 2004.

Doolittle got the win to improve to 2-0 overall, 1-0 with Washington, after giving up two runs in one inning.

De Aza, who entered the game in the third inning, broke a tie with an RBI triple in the seventh, and Adam Lind added a sacrifice fly to give Washington a 4-2 lead.

De Aza tripled down the left field line off reliever Dovydas Neverauskas, who gave up two runs in one-third of an inning. Lind followed with a sacrifice fly off Jack Leathersich for an insurance run.

Murphy had three hits and Wilmer Difo added two for Washington. Starling Marte was 2-for-4 for the Pirates.

Washington (96-63) can set a franchise record for victories if it wins the final three games of the series. The Pirates (73-86) had won four in a row after losing seven of eight games.

The Nationals took a 1-0 lead on Murphy's RBI single in the first.

Pittsburgh responded with two runs in the third as former Washington minor leaguer Christopher Bostick had an RBI double and Marte followed with an RBI single.

Left fielder Howie Kendrick dived for Marte's line drive and left the game. He was replaced by De Aza. Baker said Kendrick was dealing with hamstring issues before he dived for the ball.

Pirates right-handed starter Ivan Nova left the game in the top of the third inning after the ball hit him on his right index finger as he fouled off a bunt attempt.

X-rays were negative, Pirates manager Clint Hurdle said.

"Everything taken into consideration, we are good to go. He started and finished (the year) strong," Hurdle said of Nova. "Good for him."

Nova was replaced by pinch hitter Steven Brault, who inherited a 1-1 count and then grounded out for the second out. Nova gave up one run on four hits in two innings, and Tyler Glasnow took over in the third.

"It was a battle. He went out there and competed," Hurdle said of Glasnow, who gave up one run in 3 2/3 innings. "He threw a nice percentage of first-pitch strikes. The velocity was firm."

Lind had an RBI groundout to tie the game at 2 in the last of the third.

NOTE: Washington RF Bryce Harper got the night off after he played the previous two games. He came off the disabled list Tuesday after missing 41 games with a hyperextended left knee. ... Pirates SS Jordy Mercer (knee) was out of the lineup for the fourth game in a row. ... Washington RHP Stephen Strasburg (14-4, 2.63 ERA) will face Pirates RHP Gerrit Cole (12-11, 4.23) on Friday.