By Jill Martin

CNN

(CNN) -- Isaiah Thomas' shoulders heaved. He sat on the Boston Celtics sideline at TD Garden, sobbing.

It was Easter Sunday, and the Celtics, led by the 28-year-old Thomas, were set to start their NBA playoff run at TD Garden in Boston. His team had earned the No. 1 seed in the Eastern Conference and was preparing to face the eighth-seeded Chicago Bulls in the first round.

But the game all but paled into insignificance. Just one day earlier, Thomas' younger sister, Chyna Thomas, was killed in a single-car crash in Washington state. She was 22 years old. According to police, just after 5 a.m. local time April 15, her vehicle drifted onto the left shoulder of Interstate 5 and slammed into a pole. She was pronounced dead at the scene.

Tipoff for Game 1 wasn't far way, but the Celtics' point guard continued to cry. Teammates came by to put an arm around him or give him a hug.

It was a heartbreaking sight. Would their point guard play? Could he play?

"I never could have imagined a day where my little sister, Chyna, wouldn't be here," Thomas later said in a statement. "She and my family are everything to me, so the pain I am feeling right now is impossible to put into words. This has been without question the hardest week of my life."

'RIP Lil Sis'

Even though he's one of the shortest players in the NBA -- he's listed at 5 feet, 9 inches -- Thomas regularly comes up big. It started almost immediately with his namesake.

Thomas was named after NBA legend Isiah Thomas when his father, James, lost a friendly wager on a Lakers vs. Pistons playoff game in 1989. He had the famous name, but because of his height, Thomas had to prove to the league that he belonged. He played three years for the University of Washington and was selected by the Sacramento Kings as the 60th and final pick in the 2011 NBA Draft.

The Kings traded Thomas to the Phoenix Suns in 2014, and then he was sent to Boston as part of a trade in 2015. He has thrived with the Celtics, leading the team in scoring, and is a two-time NBA All-Star.

It was a long road to success. But Thomas had never experienced adversity like this.

A day after the tragedy, the TD Garden lights went dark for player introductions ahead of Game 1. A moment of silence was held for Chyna Thomas as her brother continued to wipe tears from his face.

And then, with the words "Chyna," "I love you," "4-15-17" and "RIP Lil Sis" visible on his shoes, Thomas took the court. And he didn't just play. He couldn't be stopped.

Thomas led Boston with 33 points and the crowd roared every time he scored.

The Celtics didn't win Game 1 that night, losing to the Bulls 106-102. They also lost Game 2, 111-97, as Thomas had a team-high 20 points.

But despite the devastating loss of his sister, Thomas hasn't missed any games -- he flew out to Tacoma, Washington, to be with his family between Games 2 and 3 -- while giving his all for the Celtics each night.

Boston clawed back into the series, winning Game 3, 104-87, thanks in part to his 16 points and nine assists.

And on Sunday in Chicago, the Celtics, led by Thomas' 33 points and seven assists, defeated the Bulls 104-95 to even the best-of-seven series at 2-2.

"Mentally and emotionally I'm not here," Thomas said following Sunday's win. "I just feed off what the guys give me. They give me a lot of confidence, so I can't do without those guys. They believe in me, and being here is what makes me, I guess, sane and makes me feel somewhat normal through this tough time."

'What he's done has been remarkable'

After Game 4, Celtics head coach Brad Stevens shook his head as he marveled on how Thomas has handled the last week.

"I can't believe it," Stevens said. "I tried to say that earlier this week, but what he's been through and the day to day is just, it's unfathomable the way that he's performed on the court. I mean, it's been really incredible. It didn't go our way in Boston, but the guys were resilient enough to come back and tie this up. But we've got a long way to go. What he's done has been remarkable."

Stevens said his players had Monday off to get ready for Game 5, which is Wednesday in Boston. The Celtics need two more wins to advance. But they have one of the best players in the NBA in Thomas, and he has the support from his teammates as he grieves.

"They've been really important," Thomas said. "I mean, they've been everything. This organization is the best organization there is, from the people who work in the Garden to the players, the coaches, everybody who is a part of the Celtics. They've been supportive. The whole world has, really. I can't say enough about that."