The Competition and Consumer Commission of Singapore (CCCS) had threatened to overturn the deal and called for changes to be made as it infringed competition rules.
Grab- Southeast Asia's dominant ride-hailing firm, operating in eight countries said it has submitted a written response to the commission defending the transaction.
"Grab disagrees with the CCCS finding that the Grab/Uber deal has led to a substantial lessening of competition," the company said.
It said some of the measures proposed by the commission were "unwarranted" but vowed it would continue to cooperate with the watchdog in their ongoing review.
Singapore-headquartered Grab in March agreed to buy Uber's ride-hailing and food business in Southeast Asia, ending a bruising battle between the ride-hailing companies.
However, Singapore's competition commission found that the deal created a virtual monopoly in the city-state's ride-hailing market, with Grab raising prices after the merger was completed. It asked Grab to revert to pre-merger pricing and end its exclusive contracts with drivers so other players would find it easier to enter the market.
Grab however said this was "one-sided" as the commission allows other industry players to enter into exclusivity arrangements. "Grab believes that this double-standard goes against the spirit of increasing choices for drivers and riders," it said.
Grab also said it has maintained its pricing and driver commissions, citing data showing that the average fare per ride fell by 3.4 percent since the merger.
The Grab-Uber deal has come under scrutiny across the region, with Malaysia and the Philippines also launching investigations.
In return for selling its Southeast Asian ride-hailing and food operations, California-headquartered Uber received a 27.5 percent stake in Grab.
Since the merger, several new players, including India's Jugnoo and Singapore-based Ryde, have entered Singapore's ride-hailing market. Indonesian firm Go-Jek has also said it plans to launch in the city-state.