Just when we thought we’d seen all Isaiah Thomas could do, the diminutive Celtics star found yet another level Tuesday, scoring 53 on his late sister’s birthday to give Boston a 2-0 second-round series lead over the Wizards.

The million-dollar question: How? How does a guy who’s 5-foot-9, who nearly averaged 29 points per game during the regular season, continue to wreck defenses so mightily in a playoff setting when defenses are hellbent on neutralizing him?

The simple answer is that Thomas and his coaching staff have found ways to make Thomas virtually unguardable within Boston’s offensive scheme; especially since Brad Stevens tweaked his lineup in the middle of the first round, a move that gave Thomas more space to navigate.

Stevens all but eliminated Amir Johnson from the rotation, meaning Al Horford is now soaking up a ton of minutes as the team’s lone post player on the court. That switch forces defenses to guard an extra Celtic along the perimeter, which is already a tough task, since Horford is also a good shooter. With opponents stretched that thin, Thomas can make use of his game-changing quickness.

That’s especially the case in handoff scenarios, where the floor general can generate a full head of steam while his man is trying to play catch-up from behind. Thomas took more handoffs than any other player in the NBA this past regular season, with 216 plays of this nature according to Synergy Sports. And he’s only gotten better at them in the last few weeks. He has scored on 56 percent of his handoffs since the playoffs began, up from 47 percent during the season. (This would’ve been the NBA’s highest mark during the regular season among players with at least 100 handoffs.)

Having Horford as the lone Celtics big helps Thomas’s handoff game immensely. Thomas — the league’s most-blocked player — has been able to get his shots off more cleanly with fewer players in the paint. What’s more, Thomas’s newfound space has left defenders without a clear way to defend him.

After receiving a handoff, he’s a nightmare on the perimeter because of all the different options he has at his disposal. Watch how reluctant Wizards star John Wall is to chase Thomas around the screen here. He doesn’t want to risk barreling into Thomas, who is one of the league’s best players at stopping abruptly once he’s turned the corner in order to draw a 3-point shooting foul.

In the paint, Washington’s bigs have been just as concerned about how to play Thomas. Stand too far back, and you risk Thomas scoring from midrange, where he is an effective, albeit inconsistent shooter. Play too far up, and he’ll use a burst of quickness to beat you to the rim, where he’s armed with an array of twists and tricks that help him compensate for his lack of height.

Trapping Thomas with two defenders as he comes around screens is one option for opposing defenses. But Stevens’s lineup switch, which has the Celtics looking more like the Houston Rockets because of how many threes they’re taking, makes that option a more dangerous choice because an unguarded Celtic shooter is now more likely to be found on the perimeter.

All of which helps explain why the Wizards have had no answer for how to guard the shortest man on the court. That Thomas is really good also helps.