The Austrian won with confidence in the first round against fellow countryman Sebastian Ofner and followed it up with a commanding performance against Ben Shelton.
After a shaky start, the 29-year-old broke his No. 122-ranked opponent twice in each of the second and third sets to win the match.
An all-Spanish affair between Alejandro Davidovich-Fokina and Bernabe Zapata Miralles was an excellent contest of high-quality tennis.
The most eye-catching fixture of all on the opening day was the one between Dusan Lajovic and fifth seed Frances Tiafoe.
The ATP 250 competition is held in Cascais, Portugal, and is one of the many clay-court tournaments ahead of Roland Garros.
Meanwhile, Hugo Nys and Tim Puetz bag their maiden ATP Tour team title at the Millennium Estoril Open.
Elsewhere, Cameron Norrie downs Marin Cilic in a tight contest to enter Estoril final.
Sander Gille and Joran Vliegen overcome a marathon battle to reach the BMW Open final in Munich.
Meanwhile, Wesley Koolhof and Kevin Krawietz advanced to the BMW Open final in Munich.
Djokovic, who has only played four tournaments this year, suffered a surprise defeat to Aslan Karatsev in semifinals of the Serbia Open last week