Heather Watson and Judy Murray were among those to offer support to Elena Baltacha following her diagnosis of liver cancer.
The former British number one, who retired from tennis in November and married coach Nino Severino the following month, announced the news on Friday afternoon.
The 30-year-old said in a statement: "I have recently been diagnosed with cancer of the liver.
"I'm currently undergoing treatment and fighting this illness with everything I have."
Messages of support quickly flooded in and Watson added her voice to the chorus of well wishes following her defeat by Agnieszka Radwanska at the BNP Paribas Open.
The British number three said: "It's terrible news to hear, and it must be so tough for her. I just wish her all the best. A good recovery. We'll all be thinking of her.
"I played doubles with her and won in Fed Cup in Sweden. I have been around her a lot. And especially being British and British tennis, all of us girls are very close.
"We will be sending our thoughts and prayers out to her. She has a real fighting spirit."
Fed Cup captain Murray has known Baltacha since she was a child starting out in the game in Scotland.
Latterly the pair have worked closely together in connection with Baltacha's tennis academy in Ipswich.
Murray told http://www.lta.org.uk: "Obviously this is a tough time for Bally, Nino and her family but she is a such a determined, upbeat person and everyone around her is staying positive as well.
"I know she'll be really grateful for all the support that she's getting."
Baltacha was diagnosed at the age of 19 with primary sclerosing cholangitis, a chronic liver condition which compromises the immune system.
Ukraine-born, she won 11 singles titles, reached the third round of Wimbledon in 2002 and the same stage of the Australian Open in 2005 and 2010.
Daughter of former Ipswich, St Mirren and Inverness footballer Sergei, she reached a career-high ranking of 49 in 2010 and was British number one for 132 weeks between 2009 and 2012.
However, she struggled with injury and illness during her career, and ankle problems eventually forced her to retire.
Baltacha's long-time Fed Cup team-mate, rival and friend Anne Keothavong and current British number one Laura Robson both sent messages on Twitter.
Keothavong wrote: "She's one of the people I most admire for her courage and determination. You'll fight through this ElenaBaltacha xxx."
Robson tweeted: "Sending so many positive vibes your way Bally! You've got this xx"
Stacey Allaster, the chairman and chief executive of the WTA, expressed her support for Baltacha, saying: "The WTA family sends its thoughts and prayers to Elena and her husband Nino at this difficult time.
"Throughout her tennis career, Elena was admired for her friendly personality, her passion, energy and tenacity. We know she will use these qualities to fight this illness and we'll support her along this process in any way we can."
Baltacha can look to Great Britain Davis Cup player Ross Hutchins for inspiration.
The 29-year-old, this week appointed the new tournament director of London's AEGON Championships, returned to action earlier this year with doubles partner Colin Fleming after recovering from Hodgkin's lymphoma - a form of cancer.
Hutchins was diagnosed with the illness in December 2012 but announced he was in remission in July.