Max Scherzer takes on his old club Friday

The Nationals begin a four-game series Thursday at first-place Arizona, which took two of three from Washington in late April at Nationals Park.

Here are three things to look for in the series:

Batting order

Will Bryce Harper continue to bat leadoff?

Harper got the night off Wednesday as the Nationals lost 2-1 in San Diego, just the second loss in 10 games for Washington.

Manager Dave Martinez put Harper in the leadoff spot on May 1 and the Nationals won four straight games and seven of eight. Harper hit four homers in a stretch from May 1 through Saturday.

But Harper was 0-for-4 in the leadoff spot Tuesday and his average fell to .230. That left him hitless in 19 at-bats in a row after he hit two homers in first two at-bats May 4.

A homecoming for Scherzer

How will Washington ace Max Scherzer fare against his former team?

In his last outing, Scherzer struck out 15 batters in 6.1 innings against the Phillies on Sunday at home.

No one in baseball history had fanned 15 batters in such a short stint as a starter.

But the right-hander didn’t figure in the decision as the Nationals scored two runs in the eighth and two in the ninth to beat the Phillies 5-4.

Scherzer is slated to start on Friday in Arizona against Matt Koch (2-0, 2.13) of the D-backs.

Scherzer was drafted out of the University of Missouri in 2006 by the Diamondbacks when Mike Rizzo, now the Nationals general manager, was part of the Arizona front office.

Scherzer made his major league debut with Arizona in 2008 and was later traded to the Detroit Tigers.

He is 6-1 with an ERA of 1.74 and leads the National League in wins and strikeouts.

The other pitching matchups for the series are Washington’s Tanner Roark (2-3, 3.65) against Arizona’s Zack Greinke (3-2, 4.10) on Thursday; Washington’s Stephen Strasburg (4-3, 3.52) against an undecided pitcher on Saturday; and Jeremy Hellickson (1-0, 2.28) of the Nationals against Zack Godley (4-2, 3.83) of Arizona on Sunday.

Arizona starter pitcher Robbie Ray, a former Nationals minor leaguer, went on the disabled list April 30 with a strained right oblique after getting injured while pitching at Nationals Park.

Bullpen woes

Will the Nationals bullpen continue to let them down?

Nationals reliever Trevor Gott came on in the seventh inning Wednesday at San Diego in a tie game.

He gave up two hits and a run and was the loser in a 2-1 setback to the Padres. It was just the second loss in 10 games for the Nationals.

That left the Nationals relievers with a record of 3-8, the worst percentage and lowest number of wins in the National League.

The Washington relievers had an ERA of 4.66, 13th out of 15 teams in the league. The Washington bullpen had allowed 16 homers, and only three teams in the National League had given up more.

For comparison, the Arizona bullpen had the best ERA in the league at 2.45 and had given up just seven homers.