Tsitsipas, 25, came out of a final at the ATP 500 in Barcelona last Sunday, a week after his coronation at the Masters 1000 in Monte-Carlo. However, he was unable to resist Monteiro, 29, who was already playing his fourth match in this tournament. Before arriving in Madrid, the Brazilian had only won one match (compared to five defeats) since March 9.
Last year, Tsitsipas reached the quarter-finals of the Madrid tournament.
In the round of 16, Monteiro will continue his journey against the Czech Jiri Lehecka (ATP 31/N.30). He took the upper hand in two sets over the Serbian promise, Hamad Medjedovic (ATP 130).
