Brazilian Monteiro beat Ukrainian world number 63 Dolgopolov 6-3 6-3 in the ATP Tour match on Sunday.
The odds shifted in favour of Monteiro, ranked 114, leading up to the match.
"The TIU was made aware of concerns over betting patterns during the match," it said.
"As with all match alerts, the TIU will assess, make a judgement and take appropriate action on the information received through its co-operative agreements with betting operators."
Dolgopolov, 28, was the favourite for the hard-court tie, but an increase in money being placed on Monteiro, 23, reversed the odds, leading to several bookmakers to stop taking bets on the first-round match.
Former top-20 player Dolgopolov, who lost his first meeting with Monteiro on clay earlier this year, failed to create a break-point chance on the Brazilian's serve as he was beaten in 55 minutes.
"I can confirm we suspended the match [betting] due to the suspicious moves in prices pre-match," said a spokesperson for betting firm Ladbrokes.
"This was flagged to relevant authorities right away."
The TIU, which was set up to police the sport, has a match alert policy that states "every alert received is assessed" and also stresses that "an alert on its own is not evidence of match-fixing".
It also lists potential other reasons to explain unusual betting patterns, including "incorrect odds-setting, well-informed betting, player fitness, fatigue and form, playing conditions and personal circumstances."
An independent review panel was set up in 2016 following a BBC and BuzzFeed News investigation that uncovered suspected illegal betting in tennis.
BBC tennis correspondent Russell Fuller
The odds shifted quite dramatically away from Dolgopolov and towards Monteiro in the hours before the start.
It is unusual for the TIU to comment directly on a match so soon after it has been played, but they do now list the number of match alerts they receive from the betting companies in a quarterly report.
There is still no firm publication date, meanwhile, for the interim report of the Independent Review Panel.
The Tennis Integrity Unit is assessing Alexandr Dolgopolov's defeat by Thiago Monteiro at the Winston-Salem Open as some bookmakers suspended their markets because of suspicious betting patterns.