PORT ST. LUCIE, Fla. -- Sometimes, opportunities surface by default.

Jon Niese was not the Mets' most successful pitcher last season -- that was R.A. Dickey, who won the National League Cy Young Award. Nor did Niese report to camp this spring with the most impressive big league resume -- that belonged to Johan Santana, who originally drew the Mets' Opening Day assignment.

April 1: Mets 11, Padres 2
W: Niese   L: Volquez
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But Dickey is now with the Blue Jays and Santana is injured, forcing Niese into Opening Day duty. Coming off the first season of his career in which he did not suffer any significant injuries, Niese could have stumbled under the weight of that new responsibility. Instead, he responded with the best spring performance of anyone on staff, giving up a total of three runs over 22 innings.

"He's been absolutely outstanding," manager Terry Collins said. "He's really prepared himself well."

When Niese takes the Citi Field mound Monday at 1:10 p.m. ET against the Padres, his wife, parents and brothers will be in attendance, watching him join the exclusive fraternity of Mets Opening Day starters. It's an honor that Niese has earned; most impressive to Collins has been the left-hander's commitment to conditioning, a significant problem for him early in his career. Struggling to maintain an ideal playing weight over his first few seasons, Niese logged significant disabled list time each year from 2009-11.

The following spring, he arrived in camp with a new nose, thanks to rhinoplasty reconstruction surgery. It served as tabloid fodder at the time, particularly because former teammate Carlos Beltran agreed to pay for the operation. But what went largely unnoticed was Niese's belief that the surgery allowed him to breathe more normally during offseason workouts, helping him redouble his cardiovascular efforts.

The result was a slimmed-down Niese, who maintained his thinner frame over the course of the summer, making 30 starts and setting career bests in innings (190 1/3), wins (13), strikeouts (155) and ERA (3.40). So when it came time to name an Opening Day starter in 2013, with Santana unavailable, the choice was obvious.

New York Mets
Projected Opening Day lineup
1 CF Jordany Valdespin
2 2B Daniel Murphy
3 3B David Wright
4 1B Ike Davis
5 RF Marlon Byrd
6 LF Lucas Duda
7 C John Buck
8 SS Ruben Tejada
9 LHP Jon Niese
Projected rotation
1 LHP Jon Niese
2 RHP Matt Harvey
3 RHP Dillon Gee
4 RHP Jeremy Hefner
5 LHP Aaron Laffey
Full 25-man roster | Depth chart

"One of the things he's done in the last few years, he's kept himself in great shape," Collins said. "Last year getting through the season healthy for the first time I thought was huge for him, and he came into Spring Training eager to show everybody that he's going to have a big season. When I told him he was going to pitch Game 1, he was truly, truly excited about it."

Heading into a year that should see the Mets debut several of their most exciting prospects, it is perhaps appropriate that Niese -- not Santana -- will throw the first pitch at Citi Field. Unlike Santana, whose contract guarantees nothing after 2013, Niese inked an extension last spring to remain under team control through 2018. Though fellow starters Matt Harvey and Zack Wheeler may boast better stuff and higher ceilings than Niese, the Mets are counting on their lone left-hander to provide rotation stability for years to come.

Niese understands all that. He realizes that when the Mets are ready to start competing for division titles on a consistent basis, they will count on him as a rotation workhorse.

"Jon has really done a good job of keeping himself in shape," said Collins, who expects Niese to eclipse 200 innings for the first time in his career. "He's got one of those durable arms. If you look back, his injuries have recently come in his legs more than everything. I think the fact that he's kept the lower half of his body in such great shape has us looking forward this year to those 220 innings."

Added Niese: "That's the way I want to do it every year."

Eager to shoot for those goals and others, Niese began silently preparing for an Opening Day start as soon as it became clear that Santana might not be ready. As February melted into March and the season drew near, Niese continued eyeing what was becoming an increasingly likely assignment.

Eventually, Collins approached Niese to officially offer him the gig. Niese replied that he would be ready.

"And he showed that he has been," Collins said.