Derek, a former NBA star and coach, took the stage with Sharna on Monday night's show and danced a jazz routine in memory of 2007, the year when his 10-month-old daughter Tatum was diagnosed with a rare form of eye cancer and had to undergo a very risky surgery.
At the time, Derek walked away from a basketball contract of over $20 million to take care of his daughter.
Once Derek and Sharna completed the dance, his daughter was shown sobbing in the audience, and Derek could barely contain his emotion after giving her a big hug.
"That was the hardest part of the night," Derek admitted Tuesday on Good Morning America when asked what it was like to see his daughter cry. "The dance part was easy. As a father, you never want to do anything -- even if it's dancing -- that makes your child cry for any reason."
"So that was tough to see," Derek continued, "but I think later in life, it will be a moment we'll both look back on and know it was very special. It was about her, and that was what the whole night was about for us."
Derek realizes, however, that his daughter was probably crying because she's grateful, not sad.
"Yeah, I mean, she's of an age now where she can understand more of what all of that meant. As we were going through it, she was an infant, so in a large way, last night was one of the big chances for her at this age to see her story and understand it in a different way," Derek explained.
"So, I think she was happy ultimately, but it's always tough to see your child cry."
On GMA, Sharna was asked which dance of the night touched her the most, but she had trouble answering.
"All of them, to be honest! I thought [Lindsey Stirling] and [Mark Ballas]'s dance was absolutely beautiful. I think creatively it was incredible and seeing just the raw emotion on Lindsey's face when she was dancing," Sharna said.
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"And obviously [Val Chmerkovskiy] and [Victoria Arlen], for me, we were all bawling our eyes out in the red room last night watching her -- and not through sadness, but through just purely being inspired by this incredible young woman. And of course [Jordan Fisher], seeing the tears on his face right before he had to dance and seeing what he was pushing through."
Sharna concluded, "It was a really incredible night, but we were all exhausted at the end of it from crying so much!"
As viewers saw this season, Derek's mother was absolutely thrilled about her son's casting on Dancing with the Stars. Derek therefore joked about how ABC wouldn't allow him to share his mother's reaction upon learning he had been booted off the show last night.
"But, you know, she's happy. I'm still her little boy," he said. "And for me to try something new and different, outside of my comfort zone -- learning how to dance a little bit with this young lady right here -- she's thrilled. She's happy."