Sinner fought off a spirited early challenge from world No.20 Shelton before taking control in a 7-6 (7-2) 6-2 6-2 flex at Rod Laver Arena on Friday night.
The 23-year-old Italian will face second-ranked German Zverev in Sunday's decider, having lost only one set en route to the final for the second straight year.
Sinner, who dismantled Australian Alex de Minaur for the loss of just six games in the quarter-finals, battled leg cramps and was forced to fight for his spot in another title decider.
Sinner was troubled by cramp in the third set before prevailing. (James Ross/AAP PHOTOS)
After suffering "dizzy spells" in the heat during his win over Holger Rune on Monday, Sinner was briefly on his haunches only 30 minutes into the clash with Shelton and was unusually demonstrative towards his coaching pod in the first set.
But Shelton let Sinner off the hook when he squandered two set points in the 12th game and faltered with his forehand in the tiebreaker.
Sinner won 11 of the last 14 points of the set and took the energy out of the contest with a dominant second set.
Left-hander Shelton had work on his right quadriceps between sets but it was Sinner who physically struggled the most in the third.
Shelton had chances to win the first set but then slid to a heavy defeat. (James Ross/AAP PHOTOS)
As cramps affected his legs, Sinner led 3-2 when he called for pickle juice and treatment at a change of ends.
The ailment led to a change of tactics as Sinner tried to keep points short and rattled off several impressive winners to help seal the match.
"A lot of tension today, slightly cramps, but also he was suffering today a little bit in his legs," Sinner said.
"I tried to move him around, tried to stay a little bit more aggressive, which today helped a lot.
"These matches can go very, very long - for three sets, two and a half hours is quite a time - so I'm happy to finish today in three.
"But I'm very, very happy to be back in the finals."
Shelton, who surprised even himself with his run to a second career grand slam semi-final, put up a gallant fight against the world No.1.
But the 22-year-old will have to wait longer in his bid to become the first American man to win a grand slam title since Andy Roddick's US Open victory in 2003.
Shelton lamented wasted chances in the first set and a poor serving performance, in which he landed just 59 per cent of first serves and won only 57 per cent of those points.
"Playing Jannik's a tough ask but I've made my living on tour so far being able to serve out sets," Shelton said.
"Having two set points on my serve, I feel like it was uncharacteristic for me not to come through and win that.
"Playing the No.1 player in the world, the chances and the windows are always small."
