Speaking in advance of this week's Arnold Palmer Invitational, Day admitted that he's concerned about Spieth embracing his role atop the world ranking.
"You know, dealing with all this stuff, it is difficult," said Day, who held the No. 1 spot for four weeks last year. "I think the biggest thing for him is to embrace the challenge of being No. 1, because if you look in the Hall of Fame there's no one out there that is terrible, and for him, I mean, he's got such a long career in front of him.
"I'm worried about him, because I don't know if he's playing too much and he's doing too many things with golf and sponsor obligations that he may get burned out and go through a rut where he doesn't want to be on the golf course for a while."
Spieth, who isn't in the field at Bay Hill this week, has already competed in seven tournaments worldwide this year. By comparison, Day, now ranked third, has played just four so far.
"I've told a few people on my team I'm kind of worried about him because of what he's kind of putting himself under," Day continued. "He has played a lot of golf, especially the last few years. You can look at his world ranking and how many events he's played over the last couple years and you can see that he's kind of wearing himself out.
"So, I mean that [is] obviously part to it, but there's just so much. Once again, there's so much that people are wanting. The timing issue is hard, because everyone wants a piece of you. I wish you could multiply yourself, because there's not enough of us to really do it."
"I'm not as popular as those guys and I understand that," he said. "Rory [and] Rickie are very popular. They're the popular kids in school. Jordan is getting that popular, starting to become a lot more popular. I'm just a nerd in the back, which is fine."