"I don‘t think I can do this": Serena Williams shares heartbreaking news 

On Friday, Serena Williams could not hold back tears as she took to Twitter to raise awareness for her friend and physio Dr. Esther Lee who is battling pancreatic cancer. The 23-time Grand Slam winner announced she has joined team Esther and is looking forward to supporting her good friend by walking in the LA Cancer Challenge 5K held on October 31.

Esther has been by both Serena and Venus' side for most of their incredible and historic careers, and is often spotted in their player boxes.

"I don‘t think I can do this, this is going to be too hard,” Serena said with tears in her eyes.

“My dear friend Esther has been a huge part of my own health journey, and now she needs our help as she battles pancreatic cancer. Esther has been named the honorary starter for the LA Cancer Challenge 5K walk-run on October 31st in UCLA, benefitting the Hirshberg Foundation."

Back in 2001, Serena opened up to press about how Esther is the reason she's still standing today. Dr. Lee supposedly urged Serena to go back to the hospital when she insisted on not going and that decision was life changing.

“I didn’t want to go and she said to me, ‘You have to go,’ and that was the beginning of things that were really bad, but ended up being good for me. “I’ve thanked her so many times for that because it literally could have been life changing," Serena said at the time.

Anyone can register to walk virtually or participate in person and make a donation here. The former No. 1 was able to laugh a little about how she will also be walking and not running, because she can't do that quite yet as she's recovering from a hamstring injury that saw her withdraw from this year's US Open.

"Let's make sure no one fights this disease alone," she said.