When i watched it back again, Massa was defending against Button through turn 11 down to turn 12, so was off line and didn't get good drive coming out of 12 and this slowed him down. Alonso took 12 normally so exited faster and caught up with Massa going down to 13. Going in to 13, Massa again goes defensive and this looks like it compromised his apex speed.......Alonso again just took turn 13 as normal and had more speed off the corner than Massa did and was able to drive past Massa on the way down to 14.
To be honest, when I saw it happen live and even the quick replay i thought Massa had slowed down and let him through, but after watching it back a couple times there, i think it was a genuine overtake.