SAN FRANCISCO -- The Washington Nationals continue to have closer issues.

Fortunately for the National League East leaders, they are no such problems with their starters.

Right-hander Tanner Roark combined with four relievers on a five-hitter Thursday night, leading the Nationals to a 4-2 victory over the San Francisco Giants in the opener of a four-game series between division leaders.

The Giants announced a trade during the game. They acquired American League All-Star infielder Eduardo Nunez from the Minnesota Twins for minor league pitcher Adalberto Mejia.

Nunez is expected to join the team in time for the second game of the series Friday night.

"He's been swinging the bat well," Giants manager Bruce Bochy said of Nunez, a .296 hitter this season, "so we're hoping he can help this offense."

Roark contributed an RBI single to a three-run second inning against Giants starter Johnny Cueto as the Nationals (60-42) extended their lead in the NL East to five games over the Miami Marlins.

"I just wanted to put the ball in play," admitted Roark, who didn't have a single hit in his first 13 starts but has totaled five in his last eight. "I want to do as much as I can to help out the team."

The Giants (59-43) lost for the 10th time in 12 games since the All-Star break and saw their lead in the NL West over the idle Los Angeles Dodgers shrink to two games, the smallest it has been since May 17.

"Man, we could've used a big hit there to really pick this club up when you're in a streak like this," Bochy said of a ninth-inning rally that ended with the potential tying run at second base. "Just couldn't quite finish it there. It would've done a lot for the club."

Roark, who had a three-game winning streak snapped in his previous start, bounced back to improve to 4-0 in his career against the Giants.

He allowed a second-inning run on a bases-loaded, infield out by Conor Gillaspie, escaped further damage by getting Cueto to fly out, then held the Giants scoreless until turning the ball over to left-hander Sammy Solis in the eighth.

Roark (10-6) allowed just four hits and the single run in seven innings. He walked three and struck out three.

"I love this ballpark," he said of AT&T Park. "There's always a big crowd and they're always hooting and hollering against us. It's always fun."

The Nationals' three-run second inning, which featured five hits off Cueto, gave Roark a 3-0 cushion before the San Francisco run in the bottom of the inning.

Cueto (13-3) was attempting to tie Nationals ace Stephen Strasburg (14-1) for the NL lead in wins.

Bryce Harper's RBI double scored Trea Turner, who had singled, to pad the Washington lead to 4-1 in the sixth.

But then the Nationals' bullpen got involved and things got interesting.

Solis pitched a hitless eighth, walking one, before closer Jonathan Papelbon allowed a one-out single to Brandon Belt and a walk to Mac Williamson in the ninth and had to be pulled.

"I'm sure he didn't like it," Nationals manager Dusty Baker said. "I had to do what I had to do to win the ballgame."

Left-hander Oliver Perez came on to walk pinch hitter Trevor Brown to load the bases. Nationals shortstop Danny Espinosa then couldn't handle third baseman Anthony Rendon's throw on pinch hitter Gregor Blanco's groundball, allowing Belt to score to make it 4-2.

With the bases still loaded and the tying run on second, Perez struck out Denard Span for the second out.

Right-hander Shawn Kelley then struck out Angel Pagan, recording his fifth save.

"We didn't have to make it that close," Baker insisted. "We've got to close these games out."

The Nationals' bullpen had blown two of the club's previous three games, allowing six runs in the final two innings in a 10-6 defeat at home to the San Diego Padres on Sunday, and five runs in the last three, including three in the ninth, in a walk-off, 7-6 loss at Cleveland on Tuesday.

"Feels good to get a win," Roark assured. "That's a good team over there."

Turner, Rendon and Ryan Zimmerman had two hits apiece for the Nationals, who out-hit the Giants 11-5 but stranded 13 baserunners.

Gillaspie had two of the Giants' five hits.

Cueto was pulled after five innings, equaling his season-low, having already thrown 101 pitches. Forty of the 101 came in the Nationals' three-run second.

The All-Star Game starter got two quick outs after Wilson Ramos' inning-opening single, but the roof caved in one piece at a time.

Zimmermann, Espinosa, Roark and Turner singled consecutively, the latter three all going to center field and each driving in a run.

"What can I say?" Cueto said. "You try to do your best and sometimes it's very frustrating that you can't throw strikes. You're not a strike-throwing matching, and I just left the strikes out there."

Cueto allowed eight hits and three runs in his five innings. He walked two and struck out three.

NOTES: Nationals LF Jayson Werth extended his streak of reaching base safely to 31 games with a fifth-inning single. ... Nationals 2B Daniel Murphy had his 13-game hitting streak come to an end on an 0-for-3 night during which the Giants twice walked him intentionally. ... Giants 2B Joe Panik (concussion) was reinstated from the disabled list before the game and immediately re-inserted into the lineup, batting third. Panik, who had missed 23 straight games, went 0-for-4. ... The Giants designated INF Grant Green for assignment to make room on the roster for Panik. ... The Giants also placed LHP Josh Osich (strained left forearm) on the 15-day DL and promoted LHP Matt Reynolds from Triple-A Sacramento.