With the words 'Chyna, R.I.P Lil Sis' written on his green left shoe, and the date of her death on the right, Thomas was in tears on the bench ahead of their playoff game against Chicago and during a moment of silence before the national anthem.
The two-times NBA All Star gathered himself once he took the court and received vocal support from the capacity 18,000-plus crowd at TD Garden in Boston.
Thomas almost lifted the Eastern Conference top seeds to victory with two late baskets in their best-of-seven first-round series opener against the Bulls, but the visitors survived to draw first blood, 106-102.
Thomas, who top-scored for the Celtics with 33 points on 10-of-18 shooting from the floor, did not talk to the media after the game but team mate Avery Bradley said the players were proud of their grieving guard.
"I know tonight he was playing for his sister, playing for his family. We appreciate that as team mates," Bradley told reporters.
"We just need to continue to fight for him ... Isaiah to me is like family. We grew up in the same area."
Center Al Horford admitted it had been a difficult situation.
"We're never going to make excuses but this is hard, this was difficult," he said. "I felt our guys, we really dealt with it the right way but Isaiah's the one who I felt dealt with it best. We knew he was hurting."
Boston coach Brad Stevens let Thomas decide whether to play on Sunday, and said that would also be the case for Tuesday's Game Two.
"Whatever he needs to do he needs to do," Stevens told reporters. "I'm not going to ask him or make him make those decisions (tonight). Those have got to come in his own time and then we will adjust accordingly."
Stevens said he was proud of the way Thomas had performed, and how his team mates had supported him.
"He's an amazing player, amazing player. He somehow plays like that ... You couldn't help but be inspired by his play."
