Asgreen got the jump on Tejay van Garderen (EF Education First) and Gianni Moscon (Team INEOS) on the 194.5km stage from Rancho Cordova to South Lake Tahoe.
“It’s my first pro win and I can’t tell you how huge this is for me, especially as it came at World Tour level and on this brutal stage," Asgreen said.
"I had a free role from the squad, and honestly, I was hoping to notch up a good result, but this is beyond anything I could have imagined before the start. I am very, very happy."
Already an overall winner in 2013, Tejay van Garderen (EF Education First) was the protagonist in the latter stages of the day with several surges designed to narrow the competition, matched only by the younger Asgreen.
“It was a good surprise,” van Garderen said. “I’m super happy, super thrilled. I’m honoured to represent yellow. I love this race. I love representing this race, coming out here and giving it everything.”
Despite falling just short of winning the stage the veteran campaigner netted himself the overall lead by six seconds on Moscon with Asgreen another second in arrears.
“The attack was partly because I didn’t want the guys behind to catch up,” van Garderen said. “I knew the steepest part was in the middle, so I knew it was kind of far out for the sprint, but I wanted to go on the steepest part.
"We ended up with just three of us, and I launched my sprint early. I couldn’t quite get the stage win, but the yellow jersey is a nice prize.”
Eight riders got out to an early break on the grinding stage which featured 4,500 metres of elevation, soaking up the early sprint and mountains bonuses. But with the road always heading skyward it became impossible for the stage to hold its shape with the break eventually swept up by the peloton and riders dropped or strung out along the route.
Chopping and changing in the lead group was eventually distilled down to 10 men with a 30 second lead on the chasers led by stage winner Asgreen.
Next up for the peloton is a 207km stretch from Stockton to Morgan Hill, one on which the new overall leader will remain watchful.
"Tomorrow we go over Mount Hamilton, and you remember what happened there in 2016, 2017? The race blew apart there," van Garderen said.
"When we go along the coast, there could be wind that day. Even though Baldy (Stage 6) is the typical, clear-cut GC day, we want to be awake for every stage.”