Nadal has spent much of a remarkably successful season sweeping opponents aside with consummate ease, but the world number one was pushed to the brink in his opening match of the prestigious end-of-year event at London's O2 Arena.
The Spaniard was playing for the first time in five weeks after resting a shoulder injury and he initially looked a little rusty as Roddick took the first set.
But the 24-year-old eventually found the remorseless rhythm that has crushed so many top players and he battled back to ensure a winning start to his bid to land the Tour Finals crown for the first time.
Nadal said: "I was a little bit lucky tonight. I started the match a little bit nervous after five weeks without playing and made too many mistakes at the beginning.
"I was very close to losing but the positive thing is I finished the match playing much better and that's good for my mentality.
"I can play with a little bit more calm in the next match and even if I lose I still have chances to be in the semis." Roddick added: "I came out of the gates aggressively and I think it caught him by surprise. He definitely wasn't settled comfortably. He was missing more balls than normal.
"The difference was he found his range on his forehand in the second and third sets.
"The two points I feel won him the match were when he hit two really good second serve returns there in the breaker."
Nadal crashed out in the group stages last year after losing all three matches and readily admits that lacklustre effort represented one of the lowest moment of his career.
His recovery since then has been stunning. In the space of 12 months, Nadal has reasserted himself as the sport's pre-eminent force by winning the French Open and Wimbledon titles, lifting a first US Open crown and returning to the top of the world rankings.
Yet Nadal's unique brand of poise and power has never translated well to indoor tournaments. He has won only one roofed event and he made a slow start here.