WASHINGTON — The Liberty had every excuse Friday to go through the motions: third game in four days; a gritty matchup in Brooklyn 24 hours earlier; travel weary after boarding a flight to D.C. in the middle of the night. 

But the true mark of a championship team is the way it goes about winning on the days when it could coast. 

Sabrina Ionescu led the charge in Friday’s 85-63 rout of the Mystics as she dropped a season-high 28 points one day after sinking a season-high six 3s against the Valkyries. 

“You’re able to go out there and see a team … who fought through fatigue, continue to play and understand there’s no excuse,” Ionescu said. “Every team has a back-to-back.” 

But she was far from the only contributor in the latest victory of the Liberty’s 6-0 start. 

Jonquel Jones returned to the lineup after missing Thursday’s game with a hamstring injury, scoring 14 points and grabbing 18 rebounds for her fourth double-double in five games this season. 

The Liberty, led by Jones’ effort, grabbed a season-high 50 rebounds.

They also held the Mystics to a season-low 28.4 percent shooting, an impressive defensive feat given their short time to prepare. 

“We don’t want a team to get anything easy. We try to blow s–t up, like Sandy [Brondello] always says, and we just try to put a lot of pressure on them,” Leonie Fiebich said, referring to the head coach. “And yeah, I mean today, in phases, we didn’t execute our defense well, especially in transition defense. I think we can do a better job there. But overall, I think in the end, we find our way.” 

Fiebich also was phenomenal, recording 12 points, nine rebounds and three blocks in what may have been the second-year forward’s most complete WNBA performance. 

Rebekah Gardner added 11 points and four rebounds off the bench. 

The Mystics are a young team, but they were nonetheless little match for the veteran-stacked, title-hungry Liberty.

The reigning champions built a 27-17 lead after the first quarter and extended it to 44-32 by halftime. 

There was a moment in the third quarter, though, when it looked as if the Liberty were slipping.

The Mystics trimmed their 12-point deficit at the half to five after scoring seven unanswered points.

By the 6:56 mark, the Mystics were within four. 

But the Liberty slammed the door on Washington with a 19-5 extended run to close out the third for a comfortable 66-48 cushion behind eight and seven points from Jones and Ionescu, respectively. 

“We have a history right now of just starting thirds kind of poorly, but the good thing is that we make our own runs,” Jones told The Post. “We get back into the game, and ultimately, we were able to put them away. So all in all, a lot of things to build upon but a win is a win and we’ll take it.” 

Some fans started heading toward the exits with 2:19 left after Ionescu stripped Jade Melbourne and Breanna Stewart made a layup to give the Liberty a 81-61 advantage. 

The Liberty have one more game during this grueling stretch of the schedule — four games in six days — when they return to Barclays Center to host the Connecticut Sun on Sunday.