Marcos will kick off his bid at the Western & Southern Open on Saturday. Appearing in the qualifying competition, he opens against Spanish veteran Feliciano Lopez.
A semifinalist in 2010, Marcos is making his eighth appearance at the ATP World Tour Masters 1000 event in Cincinnati. He owns a 10-7 record.
Against Lopez, Marcos owns a 4-1 head-to-head edge in five previous encounters. They first met in Doha in 2006, with their most recent matchup coming in Dubai two years ago. Lopez's lone victory was at Wimbledon in 2008, winning 8-6 in a dramatic fifth set.
If Marcos is to reach the main draw, he will also have to defeat the winner of Dusan Lajovic and Mitchell Krueger.