At one point a runaway leader in the event, Riiber lost 40 seconds after skiing into the wrong lane at the end of the first cross-country loop.
“It’s a silly mistake and it’s not fun to show the world that I’m maybe wasting a gold medal on that,” Riiber told reporters.
“But on a normal day with that mistake as well, I would be in the fight. I’m a good sprinter and I would fight for the gold anyway,” added Riiber, who tested positive for Covid-19 earlier in February.
His Norwegian teammate Jørgen Graabak took advantage to win the gold, with Riiber finishing in eighth.
“Of course, I wouldn’t wish that on anyone, to go wrong at the Olympics,” Graabak told reporters.
Eight years after winning the same event in Sochi 2014, Grabaak’s trademark late surge gave the Norwegian his fifth Olympic medal overall and second of the Beijing Games after winning a silver in the normal hill event.
“It demanded everything I’ve got, so I’m really, really tired at the moment. It was a really tough race, starting over two minutes behind. I didn’t think it was possible,” said Grabaak. “It’s unreal, I don’t know what to say.”
Fellow countryman Jens Lurås Oftebro claimed silver with Japan’s Akito Watabe securing the bronze medal.
Reigning men’s Nordic combined world champion Riiber had missed his opening event of the Beijing Olympics after testing positive for Covid-19.
Several teammates of Riiber had been identified as close contacts, Norway’s Olympic Committee spokesperson Gro Eide confirmed to CNN without providing names on February 4.
“The gold is yours, guys,” Riiber wrote on his Instagram account with a virus emoji and a winking face.
Those words were to prove eerily prophetic for a second time on Tuesday.
Riiber had won the ski jumping phase and took a 44-second lead into the cross-country race — just 24 hours after emerging from his quarantine hotel — with Graabak two minutes and seven seconds behind.
“I have to bring the positive thing from the ski jumping hill,” reflected Riiber. “I really showed where the potential is.
“If I bring that together with a normal cross-country race, I am unbeatable, so I just have to take the good things out of it.”
Riiber is a four-time world champion and won a silver medal at the 2018 PyeongChang Olympics in the men’s team event.