Spieth set numerous records on his way to a four-shot victory at Augusta National on Sunday, which lifted him to second in the world rankings behind McIlroy.
The 21-year-old spent Monday and Tuesday making a host of media appearances in New York, but has honoured his commitment to play in this week’s RBC Heritage event on the PGA Tour after being offered an exemption to compete in the tournament in his first year as a professional in 2013.
“(On Tuesday) my energy level was maybe at a two (out of 10). (On Wednesday) I’m back up to about a six and I should be at a nine (on Thursday),” Spieth told a pre-tournament press conference. “Fortunately I’m (playing) in the afternoon in the first round, otherwise that might have been tough.
“But I would like to get myself in contention. I don’t plan on having a hangover tournament, by any means. I would not have come here if I did not feel like I could win and there’s still a guy to catch. Rory is No 1 in the world. That is still in my mind.
“I am not as prepared as I would like to be and I won’t have hit a shot before Thursday, but I am going to practice on Wednesday and try to get myself to the shape we were a few days ago.”
Spieth’s win at Augusta came in just his ninth appearance in a major and saw him set new 36 and 54-hole scoring records, as well as equal the 72-hole record of 18 under par set by Tiger Woods in 1997.
He also became the first player ever to reach 19 under in the Masters and is the second youngest champion ever – by five months – behind Woods.
“People are talking about record-breaking and this and that and my age and stuff. To me, all that matters is that we had a goal at the beginning of the year which as a team was to prepare every part; my body, my mechanics, my mental side and our on-course efforts to peak at Augusta National,” Spieth added.
“What really matters is that our plan that we put in place was able to tie the record and set other records and that means the team that we have, some with little experience – including myself – and some with a lot of experience, the combination was the correct one and I know we can do it going forward.
“That’s what gives me a lot of confidence. To put it in perspective, we can win more because it didn’t take us long to put the right formula together to win one. That’s what’s really, really cool.”
Former US Open champion Graeme McDowell is also in the field at Hilton Head and looking for a repeat of his 2013 triumph.
McDowell finished joint 52nd in the Masters – only the third time in eight attempts he has made the cut – but knows the Harbour Town Golf Links course suits his game much better.
“A golf course like Hilton Head, which requires a lot of placement and strategy off the tee, and where I have good memories, is maybe the type of place that will switch me back on,” the Northern Irishman said.
“Switch me back on is probably a bad way to say it. If Augusta doesn’t switch you on, what is going to? But I have good memories of a golf course I know I can get round.”
Spieth is the first Masters winner since Zach Johnson to compete in the RBC Heritage the following week, and finished 12th last year and ninth in 2013.
Matt Kuchar is the defending champion after securing victory 12 months ago by holing out from a bunker for a birdie on the 72nd hole.