Apart from the celebrations of Eddy Merckx's first victory at the Tour de France half a century ago, nothing went according to plans on the race opening day.

In cycling-mad Belgium, defending champion Geraint Thomas of Team Ineos was caught in a crash on Saturday and toppled over his handlebars in the finale of the first stage. The race leader's yellow jersey ended on the shoulders of a relatively unknown rider who switched from zealous teammate to ace sprinter.

After the pile-up tore the peloton apart and played havoc with sprinters' teams, Team Jumbo–Visma's Dutch racer Mike Teunissen posted the biggest win of his career with an unexpected yet remarkable stage win at the expense of former world champion Peter Sagan, the king of sprints in recent years at the Tour.

Caleb Ewan, an up-and-coming Australian sprinter of Team Lotto Soudal, took third place on the finish line in Brussels.

Teunissen, the first Dutch rider to wear the yellow jersey since Erik Breukink 30 years ago, showed solid nerves after the second spill took out of contention his Jumbo-Visma teammate Dylan Groenewegen, the team's best sprinter.

Teunissen was initially set to be part of Groenewegen's lead-out train but quickly turned his focus to personal goals once his leader went down. In the slightly uphill section leading to the finish line on the leafy Avenue du Parc Royal, Teunissen perfectly timed his effort to deny Sagan a 12th stage win at the Tour.

"I thought all the work we had done for quite a long time was suddenly gone," he said. "But I felt good and thought I could maybe achieve a Top 5. I was relaxed, with nothing to lose. I opened up and I had a lot of power. It got me to second position and I saw Sagan was going slower than me. I threw myself at the line. Mission accomplished."

"The finale was not dangerous," Teunissen said after an impressive burst of power in the final meters that allowed him to pip Sagan. "It's only because the riders were nervous that it was dangerous."

Thomas, the former track specialist, was riding at the front of the race when the spill occurred and he bumped into barriers.

"I'm fine. It was pretty slow by the time I hit them," he said. "I gave myself enough space and avoided the actual crash but with the barriers there was nowhere to go. The main thing is that it didn't do any damage. This first week is all about just getting through."

Thomas's teammate Egan Bernal, another top contender, did not fall but was also held up by the crash. The pair did not lose time as per race regulations because the accident occurred within the final three kilometers.

Jakob Fuglsang, another favorite, also hit the tarmac about 20 kilometers from the finish in a separate crash. The Astana team leader remounted his bike with blood on his face and right knee, and tears on his jersey. He needed stitches for a cut above his right eye.

The 194.5-kilometer stage had started in a joyful mood in the heart of Brussels.

Leaving Brussels, the 176 Tour competitors started their loop south of the city at a fast tempo as a group of four riders led by Greg Van Avermaet, a one-day classics specialist from Belgium, immediately formed at the front.