He knew that the Martial Seniors he had fought thus far were not on par with high-grade Martial Seniors. Unless there were truly special circumstances, the average Martial Senior was vastly below a Martial Senior who had lived for centuries. The sheer amount of time and experience to train their body, and develop powerful techniques with immense individuality that possess great affinity with their Martial Body and Martial Art.
Furthermore, Rui was sure that pretty much all Martial Seniors had made some degree of progress with their Martial Mind, even if they hadn't reached the critical threshold.
They were immensely superior to the Martial Seniors that Rui had fought in every single parameter, he wasn't too confident if he would be able to overcome a huge gap in the power of their Martial Hearts, Bodies, and sheer quantity of techniques, experience with just his Martial Art.