Alcaraz is slated to face either Tomas Machac or Frances Tiafoe in the second round, after a bye.
Ruud will face either Giovanni Mpetshi Perricard or a qualifier in his first match.
Notable first-round matchups in Barcelona:
Arthur Fils vs Pablo Carreno Busta
Stan Wawrinka vs Alejandro Davidovich Fokina
Tomas Machac vs Frances Tiafoe
Zverev Top Seed at Munich
Alexander Zverev will look to shake off a slump on home soil in Munich, as the 2018 champion features as the top seed. He’ll face Alexandre Muller in the first round and could face sixth-seeded Jakub Mensik, the Miami champion, in the quarterfinals.
Zverev has gone stone-cold since reaching the Australian Open final in January, losing seven of his last 13 matches.
Mensik, who faces wild card Jannik Hanfmann in the first round, will play his first tennis since raising the trophy in Hard Rock Stadium.
Mensik takes a 6-4 lifetime record on clay into his first match on the surface this season.
American Ben Shelton is the second seed.
