TAMPA, Fla. -- Yankees left-hander Andy Pettitte returned to action with a bullpen session on Wednesday morning, an encouraging sign as he shoots to pitch in the third or fourth game of the regular season.

Sidelined with back spasms late last week, Pettitte threw 36 pitches plus seven warmup tosses off a mound at Legends Field in front of Yankees pitching coach Dave Eiland.

"I can just tell that I feel a whole lot better," Pettitte said. "This morning when I woke up, it loosened up nice for me. I think I'm to the point where now I have to push it and it'll start getting better.

"I know that's how it was last year when I hurt it. It's kind of scary because you don't want to set it back, but if you get through a little bit of discomfort, it loosens up and then it stays loose. I've had enough time off now where it should be on its way."

Pettitte had been envisioned as the Yankees' No. 2 starter, but back spasms pushed him back after he felt tightness driving home from Legends Field on Thursday afternoon; the second consecutive year that tightness has affected his spring. Mike Mussina will start the second game of the season in his place.

If Pettitte reports to camp on Thursday with no issues, he would expect to throw another bullpen session that day. Yankees manager Joe Girardi said that Pettitte would then pitch in a Minor League game on Saturday in Tampa, lining him up to pitch on April 3 against the Blue Jays at Yankee Stadium.

If Pettitte feels some tightness and needs an extra day, he could also pitch on Sunday in a Minor League game and then take his turn on April 4 against the Rays, with Phil Hughes making New York's third start of the year.

"It's very easy not to miss my first turn around," Pettitte said. "It's just a matter of pitching Game 3 or Game 4. We'll see how I feel tomorrow and make the call."