Chris Dobey, Ricky Evans and Luke Woodhouse had already secured their progress and were joined in the knockout stages by a clutch of big names, including Humphries.
The world No. 1 followed his nine-dart heroics on Sunday with a 5-3 victory over Nathan Aspinall to top Group A and seal his place in the next round.
Humphries only needed one leg to advance, but averaged over 102 to set up a tie with World Youth Championship runner-up Jurjen van der Velde on Wednesday.
“I knew I had to start really well,” said Humphries, who broke with a ten-dart leg and followed up with a 12-dart hold to assert early dominance over Aspinall.
“You know what you need to do. I needed one leg to go through, I knew that, but it didn’t affect me; I felt comfortable.
“It’s been a tough group, but I’m really happy to win all three matches. We’re at the business end now, and the longer format suits me. I love playing it here.”
Elsewhere in Group A, Smith beat American Alex Spellman 5-2 to reach the knockout stages for the first time since winning the tournament in 2022.
Smith, who came through last week’s Tour Card Holder Qualifier, will face Dobey in the last 16 on Wednesday.
“I’m over the moon,” said the 2022/23 world champion. “I’m through to the last 16 and I can finally relax now. I did my job getting the two points, but I’d have liked it to be 5-0 or 5-1.
“Now I’m in the last 16, I’m four games away from winning two Grand Slams. I could win this tournament, and I didn’t have that belief a couple of months ago.”
Price, a three-time champion, eased past Stefan Bellmont 5-1 to top Group D and confirm his place in the knockouts.
“I had a slow start, losing the first game, but to come through and top the group, I’m happy with that,” said Price, who has now reached the last 16 of the Grand Slam for the seventh time.
“I just wanted to qualify and get through to the last 16.”
The Welshman will face Martin Schindler in round two, as the German No. 1 advanced from the group stage for the first time in his career with a 5-2 win over Alexis Toylo.
“I’ve played Martin a couple of times. There are a lot of boys in good form, and he’s probably one of them,” Price said of his next opponent.
Woodhouse topped Group C after beating Stephen Bunting, condemning the Merseyside native to a third successive 5-4 defeat and setting up a tie against Evans.
Evans had already qualified after winning his opening two games, but went down 5-2 to James Wade, who bowed out despite the victory.
Debutant Van der Velde progressed with a 5-3 win over Damon Heta, while Dobey claimed top spot in Group B with his third straight 5-1 triumph, this time over Martin Lukeman.
The Grand Slam of Darts continues on Tuesday in Wolverhampton, with Groups E to H to be decided. Michael van Gerwen’s clash with Gary Anderson headlines three shootouts for last-16 places, while Connor Scutt meets Karel Sedlacek, and debutants Lukas Wenig and Cam Crabtree face off.
Group F remains wide open as Wessel Nijman plays Lisa Ashton before Gian van Veen takes on Josh Rock. Defending champion Luke Littler is already through but will look to top his group, alongside Danny Noppert and Niko Springer.
Results
James Wade 5-2 Ricky Evans (Group D)
Chris Dobey 5-1 Martin Lukeman (B)
Luke Woodhouse 5-4 Stephen Bunting (C)
Martin Schindler 5-2 Alexis Toylo (C)
Jurjen van der Velde 5-3 Damon Heta (B)
Gerwyn Price 5-1 Stefan Bellmont (D)
Michael Smith 5-2 Alex Spellman (A)
Luke Humphries 5-3 Nathan Aspinall (A)