He became the first Dutchman to win the Italian Grand Tour in May ahead of Nairo Quintana (Movistar) and Vincenzo Nibali (Bahrain-Merida). He and the German WorldTour team will announce his schedule at the team presentation on January 4 in Berlin.
Dutch newspaper De Telegraaf, however, confirmed on Wednesday earlier reports from La Gazzetta dello Sport that Dumoulin will return to the Giro instead of aiming at the Tour de France in 2018 as many thought.
It wrote that it "also confirms that both Team Sunweb and Tom Dumoulin have given the yes to the Giro."
Dumoulin, 27, won in 2017 thanks to his domination in both of the time trials and to his defence in the mountains against climbers like Quintana. The 2018 route contains less time trial kilometres, but still includes one 9.7-kilometre time trial stage to open the race in Jerusalem and another 34.5-kilometre time trial in the third week.
The Tour will feature a 35-kilometre team time trial and a 31-kilometre individual one before the finish.
Dumoulin, if reports prove correct, will start alongside Chris Froome (Sky), Fabio Aru (with UAE Emirates in 2018), Alejandro Valverde (Movistar), Mitchelton-Scott's Esteban Chaves and Simon Yates, and Rohan Dennis (BMC Racing). Froome's participation hinges on a salbutamol case.
Dumoulin's steady rise pointed towards a 2018 Tour ride. He nearly won the 2015 Vuelta, returned in 2016 and focused on stages to win one in the Giro and two in the Tour, and on his first planned run at the overall classification, conquered the 2017 Giro.
Insiders said that Dumoulin appeared set on the 2018 Tour with his improved climbing legs and strong time trialling, which won him the 2017 world title.
"For me it would be a surprise if Dumoulin would go to the Giro because I think he'll race the Tour, it's logical step," Dutch journalist with De Telegraaf, Raymond Kerckhoffs told Cycling Weekly earlier this month.
Those comments came after La Gazzetta dello Sport newspaper, with ties to Giro organiser RCS Sport, printed on three separate occasions that Dumoulin would participate.
"100 per cent," it wrote. "He will say 'yes' to the Giro in January. Again after the route presentation last month, "Froome and Dumoulin will be there, [Fabio] Aru too."
"The choice for the Giro appears an illogical step," De Telegraaf added on Wednesday. "Dumoulin seems ready for the fight on the highest podium in France."
Dumoulin said at the Giro presentation that he would participate in both the Giro and Tour in 2018, that he would decide on one or the other.
"It's hard to say which suits me the best," Dumoulin said. "They're both very challenging and very nice. I like both courses. They both have a little bit of everything but they're not perfect. The Giro is not perfect for me because it's lacking time trial kilometres, it's the same for the Tour.
"A perfect course would perhaps mean a less exciting race for the fans and that's not what the organisation wants. They made a nice course so that we can have a very challenging race."
The Giro's mountainous final includes the Monte Zoncolan with its 22 per cent gradient and the gravel Colle delle Finestre pass on way to Bardonecchia and the Jafferau ski station.
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