Monteiro won the match 6-3, 6-3, but in the hours leading up to the match, large amounts of money had been placed on Monteiro, shifting the odds enough that several betting agencies suspended activity.
"TIU was made aware of this situation and is following up accordingly, as it does for all alerts it receives from betting operators," the Tennis Integrity Unit said Monday in a statement. "As you will appreciate, an alert is a long way from being proof of any corrupt behavior."
Monteiro is ranked No. 155 in the world but did not face a single break point in Sunday's win.
Last week, Dolgopolov, once ranked No. 13 in the world, reached the second round of Cincinnati before falling.
"There are many reasons other than corrupt activity that can explain unusual betting patterns," according to the TIU, "such as incorrect odds-setting, well-informed betting, player fitness, fatigue and form, playing conditions and personal circumstances."
At Wimbledon earlier this year, three matches were also flagged for suspicious betting patterns.
The TIU said that if an investigation is warranted, it could take several weeks or months before it is completed.