similar theme here : topic15-21728.php
AI doesn't play any differently at end of the set or a game than at other time.
The only thing that changes AI over time is its prediction of efficiency for rushing the net ; a lot of success at net => rush more often ; not enough success => rush less often ; but it evolves all the time, it's not linked to the end of set.
Globally, you can have 3 random behaviors at the end of set : play better, play same, play worse. It's relatively easy to mentally ignore the latter 2 to say there's some forced stuff to make the 1st one happen, but it's really all just random...
1 thing to not forget : if you, as a human, play less well at end of the set, it'll lead to the CPU playing better because he'll have more time to prepare his strikes & stuff like that.