Three years before their 10-year duopoly is due to expire, Marina Bay Sands (MBS) and Resorts World Sentosa (RWS) seem to be running into headwinds.
The two casinos were granted exclusive rights in Singapore from 2007 to 2017, partly so that their operators - Las Vegas Sands and Genting Singapore - could get a head start in recouping their large investments.
These rights allowed the integrated resorts (IRs) to rev up from zero to more than US$1.2 billion (S$1.5 billion) in combined quarterly revenues in the space of a few months, and soon made the casinos the world's most profitable, with margins of about 40 per cent.
But takings have plateaued lately. Last year, the IRs' combined revenue was about $7.4 billion - a rise of just 3 per cent from that in 2012, according to estimates from UOB Kay Hian.
Macroeconomic factors partly explain the dip.
The strong Singapore dollar has kept some mass- gaming tourists away, while China's slowing economy is dampening arrivals of VIP high rollers.
However, analysts also cite fundamental issues unique to Singapore's casino market, such as the unusually rigid restrictions on local play here.
Casino demand here is not expected to show significant new growth, unless there is a steep pickup in China's economy - and hence, in inbound rich Chinese players - or unless Singapore eases gaming rules, which is unlikely.
As a result, some analysts are trimming their expectations for gross gaming revenues for the full year.
Citi Research cut its forecast for the IRs' combined revenue this year to US$6.5 billion from US$6.6 billion, mainly due to weak VIP volumes at MBS.
Although both casinos are generating significant cash flow, their own operators as well as investors no longer see Singapore as a market with good growth potential. A UOB Kay Hian analyst described it as "uninspiring".
Given the regulators' clampdown on problem gambling and strict gaming rules since 2012, a third casino also looks unlikely to brighten up the domestic scene.