Since the April 15 crash, Thomas has produced several inspiring performances, most recently a 53-point outing on Tuesday, a career high that gave the Eastern Conference's top-seeded Celtics a 2-0 lead in their best-of-seven semi-final series with the Washington Wizards.
Asked where he found the inspiration to play the best basketball of his career, Thomas did not hesitate.
"It's my sister, it's her birthday today - happy birthday - she would have been 23 today," he said on the court in Boston. "Everything I do is for her and she is watching over me, so that's all her."
Thomas said basketball is a welcome distraction from the real world.
"The only thing about it is that when I leave this gym, I hit reality and she's not here. So that's the tough part. But when I'm in this arena, I can lock in and I know everything I do is for her."
His coaches and team mates said they were in awe of how Thomas has responded to adversity, including having a front tooth dislodged during the Game 1 win in the series, where he scored 33 points.
The injury led to hours of dental surgery on Monday and Tuesday but did not sideline him for Tuesday's game.
Read the full article on eurosport.co.uk: Celtics' Thomas draws inspiration from sister's death