SAN DIEGO -- Scott Cassidy walked through spring training on a razor's edge. Residing in that class of players between raw minor-leaguers who migrate to Arizona to learn the ropes and established veterans who go to sharpen their golf games, the Padres' reliever took the mound every time knowing that every pitch would be scrutinized.

After a brief audition with the Padres last September, Cassidy spent all of March trying to prove that he could handle a full-time role at the back of the club's bullpen. And it was certainly the back, as evidenced by the fact that Cassidy earned a spot on the Padres' Opening Day roster, only to be shipped to Triple-A that same day to create space for catcher Rob Bowen.

"That was disappointing, obviously," Cassidy said.

Three days later, Cassidy received a welcome call to return to San Diego when the Padres received the unwelcome news about pitcher Shawn Estes' strained elbow. Based on his pitching since being recalled, Cassidy can finally breathe a little easier -- he won't be going back to the Portland Beavers anytime soon.

Since allowing a home run to Colorado's Garrett Atkins in his first game of the season, on April 7, Cassidy has not been scored upon in nine appearances. His streak of 11 1/3 scoreless innings is the longest for a Padres pitcher this season. On the three occasions he has been asked to throw two or more innings, the 30-year-old right-hander has given up a total of one hit with three walks and 10 strikeouts.

"What a great job he's done. He's been a nice surprise for us," Padres manager Bruce Bochy said. "When you make changes like we have in the bullpen, you need somebody to come through for you, and he has."

The Padres retained the heart and soul of their stellar 2005 relief corps in closer Trevor Hoffman and setup man Scott Linebrink, but they disposed of complementary parts such as Akinori Otsuka, Rudy Seanez and Chris Hammond.

After free-agent acquisition Doug Brocail was removed from the picture because of heart surgery, left-hander Alan Embree and righty Clay Hensley entered the season sharing seventh-inning duty for the team.

When Dewon Brazelton proved ineffective in the starting rotation, however, Bochy replaced him with Hensley and entrusted Cassidy with Hensley's crucial bullpen job. Once largely a mop-up reliever, Cassidy is now pitching with games on the line. Last Friday, he was summoned in the 10th inning of a tie game against the New York Mets and threw two scoreless innings before the Padres won 2-1 in 14.

"I know with the loss of Hammond, Aki and Rudy last year, that was a major question -- who would pick up the slack and bridge the gap to (Linebrink and Hoffman)? I think we've all done a fairly decent job of doing that," said Cassidy, whose 0.79 ERA has helped the Padres rank fifth in the National League in bullpen ERA (3.42). "Granted, we'd like to have a better record, but we've done what we can so far."

Cassidy, a former undrafted free agent who was acquired in a trade with the Boston Red Sox for outfielder Adam Hyzdu last July, does not possess the sheer velocity of many relievers. What he does have is outstanding command of the strike zone and a fastball of ordinary speed but uncommon deception.

Allow Brian Sweeney, a fellow Padres reliever and New York native (Sweeney is from Yonkers, Cassidy from Syracuse), to explain.

"He has what I call the 'Dark One,' " Sweeney said. "He throws a fastball that just gets by you. It may be 90 on the gun, but it seems to the hitters like it's 95. When he's pitching, you see a lot of swings and misses on his fastball. He's got the invisi-ball. It's a ball that the hitters can't pick up. It's part of his mechanics and his delivery."

Asked about his "invisible" pitch, Cassidy played it humble.

"I don't know how invisible it is sometimes when people get a hold of it," he said. "I've heard that there may be a little deception to it, that it gets on hitters a little bit quicker, but it's only coming in 88 miles an hour. I don't know exactly what it is. It's not like I'm doing something in particular to hide the ball."

Bochy praised Cassidy's ability to consistently throw his mysterious fastball low in the hitting zone, which allows him to rack up strikes without giving batters good pitches to drive. Once Cassidy gets ahead in the count, he tries to finish them off with his slider and change-up.

"But for the most part," he said, "I'm going to go right at the hitters."

The approach is working, so much so that the Padres might have found a bullpen fixture in a guy who was once bumped for a third-string catcher.

"To see him be successful is a credit to him," Sweeney said. "(The demotion) didn't faze him at all. He knows he belongs here. He just got on the mound and said, 'This is my job.' It's a beautiful thing to watch."

Contact staff writer Brian Hiro at b_hiro@hotmail.com.