American tennis icon Serena Williams broke down in tears while revealing her close friend and physio Esther Lee was battling pancreatic cancer.

In a 92-second clip posted to Twitter, the 23-time grand slam singles champion struggled to contain her emotions while announcing she would be participating in the LA Cancer Challenge 5K to help raise awareness for pancreatic cancer.

Dr Lee, who has served as the physio for both Williams sisters throughout their stellar careers, is regularly spotted courtside at their matches."I don't think I can do this, this is going to be too hard," Williams said, wiping away tears.

"Last year my friend Esther was diagnosed with pancreatic cancer, and she's been by my side whenever I've needed her.

"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 31 in UCLA, benefiting the Hirshberg Foundation for pancreatic cancer research.

"My family and I have joined Team Esther to help support everyone fighting this disease.

"Please join us as we raise awareness and race towards a cure by finding and advancing critical research."

As reported by Essentially Sports, Williams once admitted she could have died if not for Esther advising her to go to hospital back in 2001.

"I didn't want to go and she told me, 'You got to go', and that was the beginning of stuff that was really bad but ended up being good for me," Williams explained at the time.

"I've thanked her so many times for that because it literally could have been life-changing."