The Dutchman placed second to Chris Froome (Sky) in the Giro d'Italia in May, one year after winning the Italian Grand Tour. He then maintained that form at a consistent level, almost more than his British rival, going through the Tour de France.
"Being second in the Tour de France is already an achievement in my eyes," Dumoulin said. "But of course, I'd like to go for a win one day, but then we just have to look at what's possible and how to improve.
"No one expected me to do so well, including myself in this Tour de France. We also didn't have an eye on the podium, we were just going for it, of course we were fighting for the GC, that was a goal, but we have to look at doing better in the next years."
The world champion won the 31-kilometre time trial by one second over Froome and maintained his second place overall, at 1-51 minutes behind winner Geraint Thomas (Sky). The race ends with a flat stage in Paris on Sunday.
It has been two months since the pink confetti fell in Rome, where Dumoulin closed the Giro d'Italia second to Froome at 46 seconds back. Everyone's attention was on the double, could Chris Froome be the first since 1998 win the Tour after the Giro?
Watch: Tour de France 2018 stage 20 highlights
Froome did not have his best form, but Dumoulin timed his training well leading to the Tour. A broken wheel and time penalty cost him on the Mûr de Bretagne stage. Besides that time loss, he rode strongly throughout, over the cobbles of Paris-Roubaix, the Alps and the Pyrénées. Geraint Thomas, however, could not be stopped.
"Thomas held back because he had some time, he played a safe game. If I was closer then he would have tried to gain time on me because he was definitely stronger," Dumoulin said. "Thomas was the absolute strongest over the last three weeks. He showed that every day. Losing time on the Mûr de Bretagne didn't change anything."
Dumoulin will not race both the Giro d'Italia and Tour de France in 2019, he will aim for one with the Tour more likely.
"I'm only 27-years-old, so I find myself too young to do two Grand Tours every year, so I will probably only do one. In the last two years, it's been my biggest focus on the Giro, it'd be logical to focus on the Tour de France next year, but it depends on the parcours.
"If it doesn't suit me, then I'm more than happy to go back to the Giro because I love the race. It's not for sure I will race the Tour next year, but I'm sure I'm not going to focus on two Grand Tours next year," Dumoulin added.
"The Giro was no preparation for the Tour, it was a race on its own. I was never in my mind thinking about the Tour de France result. I had my biggest focus on the Giro this year and then went to the Tour de France."
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