A hole-out eagle on the par-4 11th was the highlight of Day's nine-under 63 that vaulted the Australian No.1 to within four shots of third-round leader Wyndham Clark.
But a foul weather forecast for Sunday threatens to reduce the $US20 million ($A30 million) event to a three-round contest.
"I keep hearing that," Day said after also collecting seven birdies in his best round of the year so far.
"I'm hoping not. If I was Wyndham, he would be probably hoping yes.
"That's just part of being at Pebble. We've had some 54-hole events (before). I remember Dustin Johnson winning a 54-hole event here one year.
"It would be nice to be able to go out there and just try and give myself an opportunity at winning, but with how the weather looks, it looks pretty average tomorrow."
A poor second-round 71 left Day playing catch-up on Saturday after starting out seven shots behind the joint halfway leaders.
"I was pretty angry with myself for how I played yesterday," said the former world No.1.
"I was first, second group off No.1 today so we were middle of the pack and I'm just going out there trying to shoot somewhere in the 60s.
"I got off to a hot start and things kind of rolled.
"I'm hoping we play 72 holes, that would be nice, especially because it's a Signature event and you want to finish the tournament.
"It would be disappointing to play 54 holes."
After making four consecutive birdies to open his round, then adding another four plus his eagle on the back nine, Day charged to 13 under for the tournament to be tied for sixth with Americans Tom Hoge (66), Justin Thomas (68) and world No.1 Scottie Scheffler (70).
Clark, though, is sitting pretty at 17 under after posting a course-record round of 60.
The reigning US Open champion's 12-under score included two eagles - on the par-5 second and sixth holes - plus nine birdies and a lone bogey on the par-3 12th.
Clark is one stroke ahead of Swedish sensation Ludvig Aberg (67) and two clear of Frenchman Matthieu Pavon, who posted a 66 in his pursuit of a second straight victory following last week's Farmers Insurance Open win.
Belgian Thomas Detry (69) and American Mark Hubbard (65) share fourth at 14 under, three shots off the pace.
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