Isaiah Thomas was always fast with his feet. When he decided to leave the University of Washington for the NBA, scouts remarked about his “exceptional first step” and ability to change direction, a necessity for an undersized point guard trying to hold his own in a league filled with bigger players.

In an NBA era that features more space than ever, Thomas’s speed allowed him to flourish. He finished on the NBA’s All-Rookie Second Team following his first season and helped earn him a lucrative second contract. Now in Boston, Thomas may be better than ever, again propelled forward by his signature asset — his speed. Only this type of speed was slightly different.

At 5-foot-9, Thomas, one of the NBA’s shortest players, became one of the league’s best this season because he sped up his shot. In the process, he’s elevated himself to become one of the league’s elite scorers.

“Isaiah has improved his off-the-dribble shooting at the three point line and beyond,” explained Celtics Coach Brad Stevens in an interview with The Washington Post. “People would say you needed to shoot it as high as you could to get off your shot most proficiently. With little guys, how quickly they get it off is more important than how high they shoot it. That’s something you see with [Stephen] Curry, you see it with [Damian] Lillard, you see it with Isaiah and others. Some of the better shooters at smaller sizes just get that thing off quick.”

Thomas’s rise to NBA stardom started last season, his first full campaign in Boston. Thomas was one of only six players in the league to average at least 22 points, six assists and three rebounds per game. Four of the others — LeBron James, Curry, Russell Westbrook and Lillard — made one of the three All-NBA teams that year. The fifth, James Harden, was the 2015 MVP runner-up and one of the favorites for this year’s award.

This season, Thomas is attempting more field goals per game within two seconds of getting the ball and more shots off the catch-and-shoot, further boosting his numbers. He is second in scoring (29 points per game) with career-highs in three-point shooting (38.4 percent) and true shooting percentage (61.7 percent) while playing 34.2 minutes per night with a huge chip on his shoulder.

“I have to. I’ve been counted out my whole life,” Thomas said. “My ability has always been questioned, no matter what level I have been on. I keep just trying to prove people wrong. I love doubt. When people doubt me I rise to the occasion.”

Thomas is as efficient as he is dangerous, with a blend of dexterity, speed and craftiness that demoralizes a defense. Step up to him on the perimeter and he will drive past you to the rim. Try to cut him off and he will bury the jumper. When the Celtics compound the issue by adding a second player in a pick-and-roll, and the Celtics offense soars to new heights.

As of Tuesday morning, he ranked first in points off drives (9.7) and is tied with Goran Dragic in number of drives per game (13.5), sixth in three-point shots made per game (3.1), seventh in free throw attempts per game (8.5) and first in points per possession on the pick-and-roll, including passes (1.09).

“He is just relentless. Fearless,” Celtics forward Amir Johnson said of Thomas. “He isn’t scared of the challenge, especially in fourth quarters. He’s just been amazing for us.”

Thomas has come up big for Boston this season. He leads the league in fourth-quarter points (10.1 per game) and is second in “clutch” scoring, averaging five points per game with the score within five points or less in the final five minutes. His offensive rating in those situations is 122.7, second best in the league.

“The game is slowing down for me,” Thomas said. “I’m just being aggressive, I’m trying to get to the free throw line, trying to make plays for my teammates. I’m in a really good zone right now. A really good rhythm.”

Having such a dynamic player also opens up the offense for everyone else.

Boston has two big men, Al Horford and Kelly Olynyk, plus small forward Jae Crowder who are no strangers to the three-point shot, making a double-team on Thomas more dangerous because he can dish it out to the open man on the perimeter. That’s especially true if that pass is to Crowder, who is 37-for-72 (51.4 percent) this season on wide-open three-point shots. Olynyk and Horford make 41.7 and 34.7 percent of their wide-open threes, respectively.

“Isaiah gets so much attention it really just frees everyone else up,” Horford said. “Teams lock in on him a lot, and try to double team him, then it is time for us to make plays. It just makes the game a lot easier.”

Because of Thomas’s speed, the Celtics are moving forward. Fast.