Roglic starts Tour of Basque Country with strong time trial
The Bora–Hansgrohe rider beat out general classification contenders Remco Evenepoel and Jonas Vingegaard by 11 and 15 seconds respectively.
Slovenian veteran Roglic eased concerns over his form after a below par showing in Paris-Nice in March by setting a time of 12 minutes 34 seconds on the 10-kilometre run starting and finishing in Irun.
Briton Tom Pidcock (Ineos Grenadiers) withdrew from the race before the time trial began after crashing during a recon run and was taken to hospital for examination.
Many of the favourites chose to compete in the first hour because of rain forecast later in the day, with the last departure scheduled for 1517 GMT.
Tuesday's stage two takes riders 160km from Irun to Kanbo in France, with an elevation gain of 2,300 metres.
The race should reveal the form of the top general classification cyclists ahead of the grand tours, except for Tadej Pogacar, who won the Tour of Catalonia in March and is not competing.