1. You don't have to be 6' tall to hit 125 mph first serves. MichaelChang, at his last or next to last FrenchOpen, hit well over 125 on ONE serve. Chang was closer to 5'7" than his posted "5'9".
2. Playing tennis for 39 years allows me to see some wierd things. YES, I"ve seen C level, or 3.5 level players, serving well over 125 mph. And they got it in once every 10 tries.
3. LowellBarnhardt, at just under 6' tall, won the 1978 GoldenGateway Fast serve competition in the amatuer class with a 129 something. He was a B, or 4.5 level player. If you look at who came in second, it would be a guy named Lee, who is 5'11" and 140lbs at the time, at 129.4. THREE of my seven tries went over 120mph, 4 didn't go in. I was still technically a C player at the time, as they didn't upgrade my rating after I won a big draw C tourney the week before.
4. Nadal doesn't want to hit over 125, because that would lower his percentange, and his game depends on attrition of the opponent, so he needs his first serves to go IN, putting more pressure on his opponent by not giving them a second serve chance.
5. Little guys like Hewitt, and Kohlshreiber have been timed in the lowest 130's on their FASTEST serves.
6. One JohanKriek, all 5'8" of him, was hitting first serves faster than the entire Wilson PRO mens team back in the '80's.