The EF Education-Nippo rider was too fast for Matej Mohoric and Mikkel Honore at the finish to collect the milestone win.
Powless’ triumph came after the trio of riders, as well as Lorenzo Rota, broke from the peloton 35 kilometres from the finish on the descent of Erlaitz.
Despite being affected by a late crash on the run-in to the finish, which saw Rota distanced from the others, the leaders were able to avoid being caught by the small group of favourites in pursuit.
It was an unusual race that did not play out as expected. With Honore up the road, Deceuninck-QuickStep did not chase for race favourite Julian Alaphilippe, leaving Trek-Segafredo to do most of the work instead.
At the bottom of the climb, Powless, Honore, Mohoric and Rota had more than a minute lead which proved too much for the peloton to chase down.
A crash on the run-in to the finish meant Rota was distanced before Powless powered home ahead of Honore and Mohoric.
“I can’t even put into words how much this means to me,” said a delighted Powless at the finish.
“Our director Juan Manuel [Garate] was so excited for us to come to this race. We didn’t have the favourite for the race, so we tried to ride the race smart. In the end, we played our cards perfectly. I’m so happy.
“My first professional victory. I’m just so happy to have taken it here in San Sebastian with a director in the car who’s a local. Every time I’ve raced through the Basque Country it’s been incredible with the fans, so I’m super happy to win in front of such enthusiastic crowds.
Wet conditions caused a number of crashes throughout the race with Mohiric the biggest name to fall victim to the treacherous roads.
The Slovenian misjudged a corner and had to unclip, which caused Honore to crash into a wall, and Rota to be taken down.
“I was watching the map on my Garmin,” explained Powless when asked about the incident.
“I think the guys who were in front of me were maybe too focussed on the situation of the race and not so much the road. I could see it was a sharp corner coming up that they ended up crashing in. In the end I was just happy to keep upright, and stay as fresh as possible for the end.
Mohoric managed to catch back to him 2.3km from the finish, and then Honore less than a kilometre later, but Powless was strongest in the sprint to take victory.
