One match at Roland Garros and three at Wimbledon, two of them in the qualifying tournament and one in the main draw, triggered alerts for unusual betting patterns.
The TIU says it received 53 alerts for the period April to June 2017.
Three of those were on the ATP Tour and one on the WTA Tour.
There were 20 each on the lower-level men's ATP Challenger and ITF Futures circuits and five on the ITF women's circuit.
The TIU says an alert on its own is "not evidence of match-fixing" and can be due to a number of other factors, including conditions and player fitness.
There have been 83 alerts in the first six months of 2017, down 38 on the same period in 2016.
Matches at this year's Wimbledon and French Open will be investigated by the Tennis Integrity Unit (TIU) for potential match-fixing.