In the final session of the afternoon, Pietreczko stormed in to a 2-0 lead before Chisnall clawed his way back in to contention by taking the next two sets to level the match.
After taking a 2-1 lead in the fifth, Chisnall, the 2021 semi-finalist, astonishingly missed match dart on double 16 after hitting two trebles from 143. The German pounced on a double eight to level the set at 2-2 and take the match into a tie breaker.
Chisnall fluffed two darts at a double once more as the German nailed double 16, and took the match with the same double moments later to book his place in the next round.
Searle steams past Dolan
Earlier, a clinical Searle registered just shy of a 70% checkout rate to win his second-round match 3-0 against Northern Ireland's Brendan Dolan to open the session.
Searle – the 20th seed – raced out of the blocks with a 13-dart hold and survived a break back of 109 from Dolan to take the first set.
He raised the tempo in the second with an early break, then consolidated with a superb 107 checkout on tops before breezing the set with a 100.2 average.
Dolan produced his best leg of the match to take out 48 on Searle's throw, but that proved to be a minor blip as 'Heavy Metal' reeled off three on the spin to win the tie with a 96.34 average.
No fairytale run for Sakai at Ally Pally
Following Searle's display, Andreas Harryson won a battle of the nomads as he swept past Ally Pally heart-throb Motomu Sakai 3-0.
The cool Harryson and exuberant Sakai held their first legs, and the Swede took out a monster 141 in 15 darts en route to winning the opening set.
Harryson stole a fortunate and nervy opening leg to break in set two and followed up with a clinical 92 take-out before claiming the stanza for 2-0.
Routine holds from both players took the third set to the distance, where Harryson prevailed with a clinical double 12 to remove 72.
Elswhere, Dutch 29th seed Dirk van Duijvenbode was beaten 3-2 by England's James Hurrell in a hectic battle that swung back and forth.
A cagey first set fell the way of Van Duijvenbode, with Hurrell mustering up just a 69.13 average and going 1/10 on the doubles.
Hurrell, though, seemingly woke up to steamroll the second with four 180s and a huge 115.62 average, which left his opponent stunned.
He then edged a nervy third set, which saw several breaks of throw to lead 2-1, but Van Duijvenbode surged back to force a decider.
Hurrell kicked it off with a break, then a whopping 132 finish on the way to eventually closing out the match for a superb victory, with Van Duijvenbode left to rue some costly doubling.
Saturday's 2026 PDC World Darts Championship results
Afternoon session
Andreas Harrysson 3-0 Motomu Sakai
Dirk van Duijvenbode 2-3 James Hurrell