This is often an unpopular opinion, but I've always believed very strongly, not just for tennis but for the other sports I've done in life, that beginners (not complete beginners) and intermediates are often more susceptible to small differences and kit changes than advanced players, especially if they're sensitive people generally. I did get to a fairly advanced level in one sport, sometimes competing with pros equally, and I knew I could adapt to differences quite easily by altering my technique, and the pros could even more than me. Some of those pros were adapting without even realising it, because they'd been doing the sport since a very young age and it was ingrained in their technique to chase outcomes and automatically adapt technique to get there. Often you get changes during competition too that you have to adapt to on the fly. Lower level players in a sport or activity often lack the natural ability to adapt like this, either consciously and certainly unconsciously. At tennis, I'm a below average club player, and I know that if you strung my racket differently, weighted it differently, or handed me a different racket, it would put me off. That little bit more power or spin I would have would throw me and I would lack the skill to adapt. I find this too with musical instruments - I cannot just pick up someone else's cello and play it - I've tried that and was aware I couldn't adapt quickly enough. My teacher can though - she can play anything. Same with the windsurfing I do.