Like so much of his marvelous career, Mariano Rivera’s initial baseball steps back from right knee surgery are going smoothly. “It was the first batting practice I threw this year and I am really happy with the results,’’ Rivera said following a 25-pitch outing at George M. Steinbrenner Field yesterday. “And it will get better.’’ Andy Pettitte preceded Rivera with a 24-pitch batting practice session he and manager Joe Girardi were satisfied with. “I just wanted to make it through with everything working the way it should be and it was,’’ Pettitte said. Girardi never detected Pettitte’s mechanics falling off, which is an encouraging sign. “He looked great and responded well. You don’t want to see a guy laboring through a bullpen session and he didn’t,” Girardi said of his 40-year-old third starter. Working with his familiar smooth delivery, Rivera breezed through the workout that was watched by former Yankees catcher Jorge Posada, Girardi, pitching coach Larry Rothschild, bullpen coach Mike Harkey, general manager Brian Cashman and assistant GM Billy Eppler. “I felt real good. The big thing will be to do the real thing, running to first base and fielding bunts,’’ the 43-year-old Rivera said. “I have done that on the back field but that’s not the same as a real game.’’