A couple of years ago I started occasionally playing 9 holes due to time constraints. I started doing this more and more and was shooting pretty decent scores, until I went to play 18 again. My back 9 score was terrible, while the front 9 was the same as what I had been seeing. This happened a couple more times and I realized it was fatigue showing up because I wasn't conditioned for that many holes. I now try to mix it up a lot more, and have noticed my back 9 scores are in line with my front 9 again. Just something to keep in mind.