Michigan State will play No. 4 seed Louisville in the West Regional semifinals Thursday in Phoenix.
Appling added 19 points for the Spartans (29-7), who had to scrap their way past the ninth-seeded Billikens (26-8). Saint Louis controlled the tempo but never got a complete handle on Green, Michigan State's superb senior, who added six assists.
Kwamain Mitchell scored 13 and Brian Conklin 11 for Saint Louis, back in the tournament for the first time since 2000.
This wasn't easy for the Spartans, and really, nothing has been this season.
Saint Louis, executing coach Rick Majerus' game plan to "muck it up" to perfection, was still within 55-51 when Green drove the left side and rifled a two-handed overhead pass to Appling in front of Michigan State's bench. The Spartans guard took aim, fired and had his 3-pointer drop, but only after it hit both sides of the rim and backboard before falling.
Appling's two free throws gave Michigan State a 63-55 lead, but the Billikens weren't going to let their season end without a fight.
Cody Ellis and Mike McCall Jr. made 3-pointers in the final 30 seconds for Saint Louis and Michigan State's victory wasn't assured until Mitchell's 3 with 3 seconds left was short.
As the horn sounded, Green, who had a triple-double Friday against LIU Brooklyn, raised his arms and hugged teammate Brandon Wood tightly as Majerus congratulated Michigan State coach Tom Izzo in front of the scorer's table.
There was no excessive celebrating for the Spartans, who have much bigger goals in mind.
Michigan State became the fourth Big Ten team to reach the regional semifinals, joining Ohio State, Indiana and Wisconsin.
After Michigan State lost its first two games this season to North Carolina and Duke, there were some who wondered if this Spartans squad would recover and live up to its high standards.
There's no debate anymore. Michigan State can win with any style you choose.
They beat the Billikens despite attempting fewer shots and being outscored at the free throw line.
Unwilling to let Michigan State's season end, Green took it upon himself to make sure the Spartans would dance deeper in March.
He scored seven straight and picked up an assist on Derrick Nix's bucket down low that gave the Spartans a 39-28 lead. Moments later, after officials had reviewed TV replays and determined Green had not hit Ellis in the face with an elbow, Green drove the right side and lofted a high shot that was blocked by Conklin.
Green grabbed the rebound, gathered himself and went up to convert the putback.
He wasn't going to be denied, not in his last season.
During a timeout late in the first half, Izzo angrily slammed his dry-erase clipboard to the floor as he dropped onto a stool inside the Spartans' huddle. The green and white pulled in tighter around their coach, who wasn't happy with his team, the officiating or the score.
Clutching, grabbing, doing anything they could within the rules to stay close, the Billikens had the Spartans right where they wanted them.
Green was frustrated. After one play, the Spartans star complained to the officials that he was being undercut whenever he tried to post up down low.
The Billikens were in his head.
Still, despite nine turnovers and a slim rebounding advantage, Michigan State took a 26-21 lead into the locker room at halftime after holding the Billikens to one field goal over the final 9:11 and 24 percent shooting (5 of 21) in the first 20 minutes.
The Billikens knew they would have to control the tempo and make it an ugly game to have a chance at pulling off the upset.
The first few minutes were downright grotesque.
St. Louis missed its first seven shots and Michigan State wasn't much more accurate, starting 4 of 13 as the Billkens' defense pestered and frustrated the more talented Spartans.
Down 9-5, St. Louis went on a 10-2 run in which the Billikens did a little of everything. Cory Remekun outhustled all five Spartans for a loose ball in the lane got a putback; Rob Loe threw in a jumper as the shot clock expired; and Jordair Jett dropped a 3-pointer that sent St. Louis fans bouncing out of their seats and brought a smile from Majerus.