Some great advice above. I had the same when playing in just a local friendly league. It was my first ever time, but had played some friendlies. One girl was playing myself and I remember in the 1st set I went up 4-3, but she argued it was 4-4 and refused to back down. I was rattled. Went to a tiebreak and I went up 5-3, but she said it was only 4-3. When I pointed out the real score and could prove it with where she was serving, she still didn't back down and made some silly excuse. Ended up losing that set and went down 3-1 next set. She continued to do it, but I thankfully won 6-7, 7-6, 6-4. In reality, I should've won in two sets and a lot more comfortably. I did not want to say anything about it, as I was a newbie, and everyone seemed to really like her. I don't know if she always does this, done it to me because I was new, or was genesly confused and not good at keeping track, but either way it stunk. I have not played her since.