This happened last month. I was playing is a 4.5 tournament and my opponent (really nice guy) started off the match playing with a Babolat APD. I went up 4-1 and then he switched to a 90's era Wilson midsize FPK Ultra type frame. I won the first set 6-2, and then he switched to an 80's Wilson aluminum frame, about an 80 inch frame. Suddenly he started playing better and I went down 0-2...I was thinking that I can't believe I'm going to lose this set to a guy using a 1980s aluminum frame. Luckily, I got back on track and went up 4-2, at which point he switched to an oversized Wilson Graphite Frame, not sure what it was, but it was a Wal Mart or Sports Authority type frame. He switched back to the Babolat at the end of the match, but I won the second set 6-3, I think. Anyway, the next day I asked him about it and he just said that golfers use different clubs, so he doesn't see why tennis players shouldn't use different frames.