The world No. 1 produced a perfect leg to wrap up a 5-3 win over Smith in their Group A clash, finishing with 177, 180 and a 144 checkout to claim his third nine-darter of 2025.
"It’s the weirdest game of darts I’ve ever been involved in," said Humphries, who has now won both of his opening matches.
"I didn’t feel good and couldn’t hit a treble. It all felt like hard work, and I thought the game was gone.
"To then go and hit a nine-darter... that’s why darts is one of the best sports in the world."
Humphries needs only three legs from his final group match against Nathan Aspinall to confirm his progression to the knockout stage.
Littler, meanwhile, continued his perfect start in Group E with a 5-3 victory over Connor Scutt, averaging 105 and hitting 55% of his doubles. The 18-year-old defied a 170 checkout from Scutt to make it two wins from two.
"We were both up for a good game," said Littler. "I can say now that I’m through, relax tomorrow and come back Tuesday.
"It was better than the first game, and hopefully I can play even better going forward."
Ricky Evans also progressed with a narrow 5-4 win over Stefan Bellmont. Three-time champion Gerwyn Price kept his hopes alive in the same group with a 5-0 whitewash of James Wade, a result that eliminated the 2025 UK Open and World Matchplay finalist.
Elsewhere, Chris Dobey eased past Damon Heta 5-1 to reach the last 16, while last year’s runner-up Martin Lukeman was knocked out after a 5-4 defeat to Jurjen van der Velde.
Danny Noppert also advanced thanks to a 5-4 victory over Cam Crabtree, who now faces Lukas Wenig in a decisive match, after the German beat Jonny Clayton 5-3.
Germany’s Niko Springer continued his impressive debut campaign with a 5-4 win over Michael van Gerwen, leaving the Dutch giant facing a winner-takes-all meeting with Gary Anderson.
The Scot edged out 21-year-old Beau Greaves 5-4 in a high-quality match that saw both players average over 102.
"She’s going to cause so much trouble next year on the Pro Tour," Anderson said of Greaves. "From these youngsters, there are going to be so many good players.
"I don’t think one person will dominate darts over the next five years."
Stephen Bunting became another high-profile casualty, exiting after two straight 5-4 defeats, this time to Martin Schindler. Luke Woodhouse secured progression from Group C with a 5-2 win over Alexis Toylo, who now faces Schindler for the remaining spot.
Gian van Veen averaged 107 in his 5-3 win over Wessel Nijman, while Josh Rock whitewashed Lisa Ashton 5-0 to keep Group F finely balanced.
In Group E, Karel Sedlacek missed ten match darts before beating Daryl Gurney 5-3, setting up a decisive meeting with Scutt for a place in the next round.
Aspinall’s 5-2 win over Alex Spellman leaves Group A finely poised heading into Monday’s final round of matches, which will conclude the group stages for Groups A-D. The remaining groups will finish on Tuesday.
Results
Afternoon session (Groups B, D, F, H)
Jurjen van der Velde 5-4 Martin Lukeman
Lukas Wenig 5-3 Jonny Clayton
Josh Rock 5-0 Lisa Ashton
Gerwyn Price 5-0 James Wade
Ricky Evans 5-4 Stefan Bellmont
Danny Noppert 5-4 Cam Crabtree
Gian van Veen 5-3 Wessel Nijman
Chris Dobey 5-1 Damon Heta
Evening session (Groups A, C, E, G)
Nathan Aspinall 5-2 Alex Spellman
Karel Sedlacek 5-3 Daryl Gurney
Martin Schindler 5-4 Stephen Bunting
Gary Anderson 5-4 Beau Greaves
Luke Littler 5-3 Connor Scutt
Luke Humphries 5-3 Michael Smith
Niko Springer 5-4 Michael van Gerwen
Luke Woodhouse 5-2 Alexis Toylo