American Jordan Spieth faces "very tough competition" to return to the top of the world golf rankings this year, he said on Tuesday.

The 23-year-old, who won the U.S. Masters and U.S. Open in 2015, started his campaign with a third-place finish in last week's Sony Open in Hawaii and lies fifth in the rankings.

"A great start to the year and excited for 2017. It's going to be a good year," Spieth told reporters at an event in Tokyo where he unveiled the "Spieth One" golf shoe which he has been working on with U.S. brand Under Armour.

Asked if Japan's world number six Hideki Matsuyama would be one to watch, Spieth said: "Very, very tough to beat this year."

"Justin Thomas, another friend who just won the last two tournaments, Rory McIlroy, Jason Day, Dustin Johnson - a lot of really good players that are young players in the game of golf right now, it's very, very tough competition."