At Volk's current level, I think he's below prime Nanjiro and current Nanjiro.
Explanation - Prime Nanjiro >= Ryoga right now, Ryoga > Volk due to expectation/hype. Byo didn't want to f with Ryoga even a little bit.
Explanation - Current Nanjiro, he seems still bored with tennis. Even though Nanjiro is old, I would expect him to challenge any/all rando's just to get a challenge. He doesn't appear to be out of shape.
That being said in a few years Volk should be better than current Nanjiro.
It's still insane that Ryoma is like 5 foot nothing /151 cm and 110 pounds / 50kg? And Nanjiro is 5'8 / 178 cm = probably about 225? lbs / 102? kg. Ryoma's down 8" and a 100 pounds.
Echizen Ryouma (越前(えちぜん)リョーマ) is a 1st year student at Seigaku. Ryouma is a tennis prodigy and the protagonist of The Prince of Tennis. He previously lived in the United States, where he won four consecutive titles.
tenipuri.info
The composite lightest/smallest top tennis player is 163 cm / 64 kg. 14 kg and 12 cm is an insane disadvantage when you get to the best of the best world class athletes. Logically, Ryoma is
the best tennis player that has ever lived or will ever live at maximizing the output of his muscles.