Media playback is not supported on this device
Kvitova, 26, is undergoing surgery for severe lacerations to all four fingers on her left hand - her playing hand.
The incident, described as a burglary, happened in Prostejov, Czech Republic.
"I am shaken," said Kvitova, adding: "The injury is severe and I will need to see specialists."
Her publicist Karel Tejkal said: "It was a random crime, nobody was going to attack or rob her as Petra Kvitova."
Kvitova is ranked 11 in the world and has won a career total of 19 titles, including Grand Slam victories at Wimbledon in 2011 and 2014.
It is understood she allowed someone posing as a utilities man access to her apartment and while defending herself in a struggle, suffered the injuries to her hand. The intruder is believed to have then run away and is being sought by police.
Kvitova has suffered damage to ligaments and tendons in her left hand, but the full extent of the injuries will not be known until after surgery later on Tuesday.
"Thank you for all your heart-warming messages.
"As you may have already heard, today I was attacked in my apartment by an individual with a knife. In my attempt to defend myself, I was badly injured on my left hand.
"I am shaken, but fortunate to be alive. The injury is severe and I will need to see specialists, but if you know anything about me I am strong and I will fight this. Thank you all again for your love and support and now I would appreciate some privacy while I focus on my recovery."
Kvitova had announced earlier on Tuesday that a foot injury forced her to withdraw from January's Hopman Cup in Perth, Australia.
She had only returned from pre-season training in Lanzarote last week, despite her right foot requiring a protective boot for much of the time.
It meant her hopes of playing in the first Grand Slam of the season were already doubtful, with the Australian Open getting under way on 16 January.
Kvitova had finished the 2016 season strongly, picking up titles in Wuhan and Zuhai, as well as winning the Women's Tennis Association's annual sportsmanship award, for the fourth year in succession.
This content will not work on your device, please check Javascript and cookies are enabled or update your browser
Two-time Wimbledon champion Petra Kvitova said she is "fortunate to be alive" after being injured in a potentially career-threatening knife attack at her home on Tuesday.