Northern Ireland's Rock reached the last four with scintillating wins over Gerwyn Price and Michael van Gerwen, and another shock appeared to be on the cards as the World Cup winner surged into a 5-0 lead.
The 24-year-old hit a remarkable 102.56 average to lead 6-1 but back came Littler with a stunning nine-darter - only the 10th in Matchplay history - to reduce the deficit.
The largely subdued crowd sprang into life as Rock and Littler exchanged thrilling legs, but with Rock leading 9-6, the world champion stormed back to win five legs in a row - capping the achievement with a breathless 12-darter on double 20.
Litter appeared in total control, and despite Rock completing 170 and 120 checkouts to reduce the score to 14-13, held throw on double 10 to move one leg away.
The teenager booked his maiden Matchplay final against 2007 champion James Wade - a repeat of the UK Open final, which he won in March - with his first attempt on double 16.
"An incredible game," Littler told Sky Sports. "I wasn't happy at 5-0 down. A slow start. I picked myself back up.
"I wouldn't say Josh fell off a little bit, but the first five legs from him were unbelievable. As the match went on I said to myself I could do it. I've been here before and I've done it again."
On the nine-darter, he said: "It definitely sparked me into life. I didn't want to give it big. I was playing awful. At that point I wanted to go into the break at 6-4 but I ended up 7-3 down. Once again I said to myself I could do it and come back again.
"Everyone knows about the triple crown. You can't put anything past James Wade. He's been playing some brilliant darts. I'll relax and come back tomorrow."
Wade wins 38-leg epic
In the earlier semi-final, James Wade survived an astonishing fightback from Jonny Clayton to win a 38-leg classic - the longest semi-final in the tournament's history - to reach his seventh World Matchplay final.
Wade lost four of the opening five legs in his quarter-final victory over Gian van Veen, but there were no early signs of nerves against the Welshman after storming into a 7-2 lead.
The world number eight was in imperious form losing just two throws as he surged into a 16-10 lead, and it seemed just a matter of time before he would cement his place in Sunday's final.
Yet with just one leg required, back fought Clayton who won the next six legs - including a 102 finish and 13-dart hold - to take the match to a tie-breaker.
Wade re-took the lead with a bullseye and could have sealed the win with match dart on double 18 but astonishingly missed, allowing Clayton to take double 20 to level the match at 17-17.
As both exchanged legs and with pressure growing, Wade held his nerve on double four to take a 19-18 lead and when Clayton missed double 10 the Englishman sunk a double nine for a historic win.
"I am flabbergasted," Wade said. "With darts you are surrounded by great darts players who are full of massive talent.
"Jonny let me off at the last minute, but I let him off before that.
"I'm exhausted after that. I thought that was a good game, but I look at the averages and I didn't know it was that good.
"I hope the crowd enjoyed it and everyone at home themselves - because I hated every single minute of it."