CHICAGO -- Jake Arrieta wasn't his usual dominant self. It didn't matter.

The red-hot Chicago Cubs rallied in the seventh and won the game in the 13th on Javier Baez's home run to beat the Washington Nationals 4-3 on Sunday.

The Cubs swept the four-game series and extended their season-high winning streak to seven.

Chicago has the majors' best record at 24-6 while the Nationals are 19-12.

"Our guys were into that game until the very last drop, I promise you that," Cubs manager Joe Maddon said.

The second home run of Baez's season was his first career walk-off blast. Blake Treinen (2-1) gave up the home run.

"We had no one on the bench, and we're still playing hard," Baez said.

It was also an unusual game for reigning National League MVP Bryce Harper. The Cubs walked him six times -- three times intentional -- and he was hit by a pitch in his other plate appearance.

"It's happened before to me, not at this level," Harper said. "It was definitely when I was younger, in high school and college and what not. They had a plan. They had a plan and unfortunately it worked."

Arrieta -- the reigning National League Cy Young winner -- allowed three runs (two earned) and six hits in five innings in a matchup of the league's top teams. He struck out seven, but walked four (one intentional), threw three wild pitches and committed a fielding error. His 0.84 ERA that led all of baseball rose to 1.13.

Kris Bryant's two-run single tied the game 3-3 in the seventh, preventing Arrieta's first loss since July 25.

The Nationals loaded the bases in the 10th on a single and two walks (one intentional to Harper), but Ryan Zimmerman flew out to end the threat. The Cubs also threatened in the inning, putting two on before Baez flew out.

Jason Heyward reached on an infield single and tried to score on Bryant's double in the 11th but was thrown out at the plate.

In the 12th, the Cubs walked Harper for the sixth time -- tying the major league single-game record -- to load the bases before Zimmerman grounded out.

"We did what we thought we had to do today, and it happened to work. So good for our guys," Maddon said.

Harper became the first player to draw 13 walks in a four-game series since at least 1914 and is the first player to reach base seven times without an official at-bat since at least 1913.

"It's kind of like in basketball when they have hack-a-Shaq," Nationals manager Dusty Baker said. "I guess that's part of the game and part of strategy."

Baker acknowledged that other teams might try the Cubs' strategy against Harper.

"The fans didn't come here to see him walk," Baker said. "They come to see him swing the bat."

The Nationals left 21 runners on base and went 1-for-19 with runners in scoring position.

"They'll be tough, especially to play that long and have that many chances," Baker said. "You're sitting on the edge of your seat. Had opportunities big time. Lose a game like that, it hurts, but we don't have any choice, really. You gotta lick your wounds and limp back home and gain some strength."

While Arrieta faltered, Washington starter Tanner Roark struck out seven and allowed four hits and an unearned run in six innings.

Daniel Murphy doubled and scored on Wilson Ramos' single for a 3-1 Nationals lead in the fifth.

The Cubs cut the lead to 2-1 in the fourth via an error, two groundouts and a wild pitch.

Anthony Rendon's bases-loaded groundout scored a run for a 2-0 Washington lead in the fourth.

Harper scored from first on a Zimmerman double off third baseman Bryant's glove for a Nationals' 1-0 lead in the third.

NOTES: The Cubs activated RHP Neil Ramirez (0-0, 4.50 ERA) from the bereavement list on Sunday. To make room for Ramirez, the Cubs optioned RHP Spencer Patton to Triple-A Iowa after Saturday's game against the Nationals. Patton allowed a run without recording an out in his one appearance. ... Cubs C Miguel Montero felt good and had no issues increasing his hitting on Sunday, manager Joe Maddon said. Maddon has no timeline for Montero's return. He went on the disabled list April 25 for lower-back tightness. ... Nationals 2B Daniel Murphy returned to starting lineup Sunday after he was out of the lineup Saturday due to an illness. He pinch hit Saturday.