The left aero bar on the 25-year-old Australian's time trial machine broke, forcing him to make a bike change in the final 15km that cost him valuable time as he slipped down the leader board to fifth, 1:10 behind winner Fabian Cancellara (Switzerland) and eight seconds behind bronze medalist Chris Froome (Great Britain).
Dennis, who started in the second wave of riders with most of the other medal contenders, set the fastest time check at the top of the Grumari climb on the first lap of the 29.9km circuit, with Cancellara just squeezing past him minutes later. Dennis topped Cancellara by a seemingly significant 25 seconds at the final time check, but the 2008 gold medal winner was able to make up that time and climb back into the lead by the start of the second lap.
Nevertheless, Dennis looked on track for a medal when disaster struck.
"These things happen," Dennis told the Adelaide Advertiser. "I can't be too disappointed. Obviously it's out of my control. Look, I can still be proud of how I rode up until the mechanical, changing the bike and finishing off going hard, don't give in."
Dennis took solace in the fact the he lost to a champion like Cancellara, a four-time time trial world champion and now two-time Olympic gold medalist.
"(It) was a huge ride from Fabian," Dennis said. "Hats off to him. He's an impressive rider. He's won just about every race, so he's a real champion. He rode a perfectly paced race today, and going out in his last year as Olympic champion is a huge effort."
Dennis has been focused on the Rio Olympic Games since his breakout performance in the 2015 Tour de France, when he on the opening prologue time trial and wore the first yellow jersey of the race. He faced illness earlier this year, but then recovered in time to start the Tour de France in July, abandoning the three-week Grand Tour after stage 16 to prepare for Wednesday's race.
"Physically I felt good," Dennis said. "I thought maybe I went out a bit too hard in hindsight, but you've got to throw it all out there. If I pulled it all off and didn't crack in the last third, or without the mechanical, I would have been saying it was a perfect day.
"Gold was definitely the number one goal. Once I had the mechanical it was all about a medal; don't give in."