Geraint Thomas Homecoming
Image credit: PA Sport
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The Team Sky rider had reportedly been considering the Italian race despite ruling out the move last month, alongside Chris Froome.
However, the Welshman, who pipped his team-mate to last year’s title in France, has now ruled out any tilt at the Giro.
"It was tempting, but I did not want to jeopardise the chances of the Tour for anything this year," said Thomas.
"The Giro feels like unfinished business and I wanted to think about that," said Thomas.
"It is an attractive route especially with three time trials, although they are hilly.
"Maybe next year or the year after but we will see. It has always been about the Tour this year.
"As defending champion I feel as if I have to go back and want to be in the best shape possible."
Thomas will start his 2019 season in the five-stage Volta a la Comunitat Valenciana in southern Spain from today, which is his first race since the Tour of Britain last September.