The Japanese debutant stunned the three-time world champion to take the first set 3-1 in legs, capitalising on some poor scoring from the often monstrous Van Gerwen.
The Dutch all-time great, usually so reliable on the doubles, struggled to find his finishes in the early embers, hitting just one of six attempts in the first set and a miserable three out of 17 in the second, uncharacteristically averaging under 90 in the early stages.
With Van Gerwen faltering, Tatsunami found a stunning break of throw at the start of the second leg as the Ally Pally crowd jubilantly chanted his name.
Tatsunami promptly held with a 71 checkout to put him on the verge of an unthinkable two set lead, as MVG's body language grew increasingly furious.
Double 8 was pinned to pull back a leg, but Tatsunami went on the attack knowing a hold of throw would put him on the brink of a huge 2-0 lead in sets, but unable to take out 116 on the board, Van Gerwen capitalised to set up a deciding leg with darts in hand.
Van Gerwen's scoring became erratic, going from knocking in smooth 180s to shocking low double-digits, and he missed a key chance at double 16. Still, Tatsunami couldn't respond, letting MVG in to eventually take out a double 2 to level the scores at one each.
The third set began with a break for Van Gerwen, who looked as if the walk-off had done his mental wellbeing some good. He held with a 65 checkout in the second leg, making it five on the spin as his opponent began to show signs of fatigue.
Tatsunami would claw one back on double 16, but a hold of throw would put Van Gerwen 2-1 up - which he took out in 13 darts to make it 2-1 on the scoreboard in sets.
Tatsunami turns up
Still, Tatsunami wasn't rolling over. He broke throw with an 18-dart leg to begin the fourth set after some lacklustre set up play from MVG, before holding the second leg with a 62 checkout via tops.
An efficient 14-darter from Van Gerwen brought the set back to 2-1, but the 180s were not coming as often as he's accustomed to.
Finishing turmoil then began to haunt both players, as Tatsunami narrowly missed bull for a stunning 161 finish, before both fluffed their lines in quick succession, with MVG eventually scraping through to save the set, with darts in hand for the next.
Tatsunami would somehow be on for the finish first, but missed tops with 73. MVG failed to take the match on 48, and Tatsunami again missed his opportunity.
With two set darts missed, Van Gerwen finally took out double 10 to squeeze into the second round, breathing a huge sigh of relief in the process.
Van Gerwen managed just 29 per cent on his doubles and not a single ton-plus finish, barely averaging over 90, while Tatsunami managed to pin 35 per cent of his checkouts.
'That's darts'
"That was really difficult," an emotional Van Gerwen told Sky Sports immediately afterwards.
"I don't know what happened. I started off really crap. My first darts were constantly underneath and above. Nothing really worked.
"I feel good but when you put yourself under pressure, it doesn't make it any easier.
"His finishing was great. I wasn't afraid of his scoring. When you score so bad and when you don't do what you are capable of, you get annoyed and then have to deal with your own problems.
"I kept battling my own battle. I shouldn't put myself in that position. I'm bigger than that. But that's darts."
Chissy knocks out Sherrock
Fallon Sherrock - the first female player to ever win at Ally Pally but without a victory in the tournament in the past four editions - was beaten by Dave Chisnall in straight sets as the 45-year-old bids to rediscover the form that saw him in the top 10 of the PDC rankings in 2024.
Despite keeping pace in averages, the Queen of the Palace struggled massively on the doubles, hitting just one in 15 attempts as Chissy raced to a commanding two-set lead, which included an excellent 160 checkout in the first.
The third set saw a spirited fightback from Sherrock, as she attempted to build momentum with back-to-back legs, but Chisnall - a former World Championship semi-finalist - battled on to make it 2-2 as the former women's champion again stuttered on the finishes.
Sherrock then missed a crucial tops to take the set, leaving Chisnall to take out double seven for the match win.
"That was massive," Chisnall told Sky Sports after his win.
"If she hit her doubles, that would have been very different.
"She scored well. I didn't score well at all. My doubles were quite good. Fair play to the crowd."
Chisnall, whose form has slumped this past year having picked up just two match wins at majors in 2025, averaged 89.66 to Sherrock's 84.83, but was almost 50 per cent on the doubles.
The Englishman will face Ricardo 'Pikachu' Pietreczko in round two.
Wattimena tested
In the opening encounter of the evening, Jermaine Wattimena survived a scare against young German Dominik Gruellich, who took the world number 19 to a fifth-set decider before the Dutchman ultimately powered through to set up a second round clash with Scott Williams.
After taking a first-set lead, Wattimena surprisingly found himself two sets down before recovering by rattling off six legs on the bounce in the fourth and fifth to secure the victory, averaging 87.28 and pinning four maximums along the way.
In the final match of the night, Poland's Krzysztof Ratajski stormed past Alexis Toylo of the Philippines in straight sets, landing a fine big fish break of throw in the second set to get over the line, setting up a second-round clash with England's Ryan Joyce.
Earlier in the day, Kenyan qualifier David Munyua caused the biggest upset of the PDC World Darts Championship so far in his debut at Alexandra Palace, stunning Belgium's Mike De Decker 3-2 during Thursday's afternoon session.
Japanese debutant Motomu Sakai also announced himself in emphatic fashion at the worlds, producing a standout performance to sweep past Thibault Tricole.
The qualifier overcame a lively start from Frenchman Tricole to claim a 3-0 victory, combining sharp timing on the finishes with a clear appetite for the stage.
The action continues in north London on Friday, with the likes of Josh Rock, Beau Greaves and Nathan Aspinall all stepping up to the oche.