The two players, who began dating last year, appear to have recently ended their relationship.
"To combine a career and a [relationship] between two top players is sometimes impossible," Kvitova was quoted as saying.
Both players had been in relationships with other Czech pros. Stepanek was previously married to the retired Nicole Vaidisova, after a brief engagement to Martina Hingis. Kvitova was dating a player she had known from juniors, Adam Pavlasek.
Czech tennis spokesperson, Karel Tejkal, said, "Petra and Radek are friends and will continue to support one another."