If they played in the National League East instead of the American League East, the Yankees might go undefeated.

Their season-high nine-game winning streak has come at the expense of three NL East teams, and they get to play two of them again this week. The Yankees celebrated Father’s Day in the nation’s capital with a 4-1 victory Sunday that completed a three-game sweep of the Nationals and a six-game sweep of the trip that began in Atlanta immediately after they swept Round 1 of the Subway Series last weekend at Yankee Stadium against the Mets.

The Yankees get to play the Braves and Mets again this week with the Bravos coming to the Stadium for three games beginning Monday night before the Bombers bus to Citi Field for a three-game set in Round 2 of the Subway Series that starts Friday night.

Quality pitching has fueled the Yankees’ streak with Ivan Nova’s work Sunday among the best starts over this stretch. Only a second-inning home run by Adam LaRoche marred the effort by Nova, who gave up just six other hits with one walk and four strikeouts in 7 2/3 innings as he improved his record to 9-2 with a 4.32 ERA.

Nova has gone more than a full calendar year without losing on the road. Since a defeat at the hand of the Angels June 3, 2011 at Anaheim, Nova has an unbeaten string of 15 starts on the road during which he is 12-0.

One day after finally winning a game without hitting a home run, the Yanks got long balls from Curtis Granderson (No. 21) and Robinson Cano (No. 12) to negate LaRoche’s blow. They needed the homers because once again the Yankees struggled with runners in scoring position (0-for-12). That statistic grows more and more meaningless as the Yankees keep piling on victories despite folding in the clutch.

The Yankees went unbeaten on a trip of at least two teams for the first time since May 7-12, 2002 when they won three-game series at Tampa Bay and Minnesota. They also swept three series in a row for the first time since May 5-14, 2007 against the White Sox, Pirates and Diamondbacks.

Saturday’s 5-3, 14-inning victory raised the Yankees’ record in extra-inning games to 3-0, a marked improvement over last year when they were 4-12 past regulation. It was their first victory in a game of 14 or more innings since Aug. 7, 2009 when they beat the Red Sox, 2-0, at the Stadium on a two-run home run by Alex Rodriguez. They had not won a road game of that length since June 1, 2003 at Detroit, 10-9, in 17 innings.