Background: Heart failure (HF) patients experience reduced functional fitness level (determining the performance of routine, daily activities) and diminished exercise capacity (linked to more effortful activities). Aim: The aim of the study is to assess this relationship using functional fitness tests compared to peak VO2 and VE/VCO2 slope in the context of exercise capacity and ventilatory response to exercise. Methods: A total of 382 men with stable HFrEF (age: 61 ± 10, NYHA class I/II/III/IV: 16/50/32/2%, LVEF: 30.5 ± 8.3%) underwent cardiopulmonary exercise testing (CPX) and a Senior Fitness Test (SFT). Afterwards, the patients were divided according to the 2capacity with peak VO2 ≥ 18 mL/kg/min, those with higher or lower ventilatory responses (VE/VCO2 slope ≥ 35 vs. <35) to the exercise were compared. Results: Patients who covered shorter distances in the 6 min walking test showed worse results in the functional tests (‘stand up and go’, ‘chair stand’ and ‘arm curl’) and CPX (lower peak VO2, shorter exercise time and higher VE/VCO2 slope). Subjects classified into Class D demonstrated the worst results in all elements of SFT; those in Class A demonstrated the best results. Significant differences that were analogous occurred also between classes B and C. Among the participants who reached peak VO2 ≥ 18 mL/kg/min (n = 170), those with VE/VCO2 slope ≥ 35 were characterized by worse physical fitness as compared to those with VE/VCO2 < 35. Conclusion: Reduced exercise tolerance led to worsening physical function in patients with HFrEF. Moreover, limitations in physical fitness seem to be distinctive for those patients showing excessive ventilatory response to exercise slope VE/VCO2 (≥35). The Senior Fitness Test may be considered as a useful tool for assessing comprehensive functional and clinical status and risk stratification in patients with HFrEF, especially those with extremely low exercise capacity.