The keepers at River Safari's Giant Panda Forest first noticed the peeking panda in February.
In the early mornings, male panda Kai Kai can be seen looking out for female partner Jia Jia through a gate that separates their living areas.
It is a sign that Kai Kai is "slightly more" interested in Jia Jia than before, said Dr Cheng Wen-Haur, chief life sciences officer at Wildlife Reserves Singapore. A year ago, the male panda was reportedly more into food.
While there is no telling yet when Singapore will see its first panda cub, Dr Cheng's team has been upping the game as matchmakers ahead of Jia Jia's sixth birthday on Wednesday. Pandas typically become sexually mature at age six.
The furry duo, here on a 10-year loan from China, celebrate both their birthdays and their second year in Singapore this month. Kai Kai turns seven on Sept 14.
To get the bears used to each other's scent, keepers have been swopping their dens and scattering Jia Jia's urine around Kai Kai's exhibit. The bears live separately.
In March, Kai Kai was found to be "a healthy, fertile male" after a medical examination. It has also been responding well to its daily "sexercise", or squat trainings - which are intended to strengthen its hind limbs for the act, said Dr Cheng.
However, vets who have been analysing Jia Jia's urine regularly since January to check for spikes in hormone levels have yet to find any indication that the female panda is ready to mate.
It is a race against the clock. Female giant pandas like Jia Jia have only one reproductive cycle a year. There is a window of just two to three days when a female panda is willing to mate, and it is fertile for only half that period.
An added complication is that first-time breeder Kai Kai might not know how to mate.
The difficulty in breeding pandas is one reason that giant pandas are an endangered species. There are only 1,600 left in the wild in China and 300 in captivity around the world.
Even so, there are encouraging developments - a mother panda delivered triplets at the Chimelong Safari Park in Guangzhou on July 29.
Meanwhile, vets and keepers are closely monitoring the pandas for significant changes in behaviour, said Dr Cheng.
"When they are in heat, male pandas vocalise and perform handstands against trees, walls and rocks, scent-marking as high up as possible," he said.
"Females show signs such as scent-marking, restlessness and characteristic bleating sounds."
When the time comes, Kai Kai will no longer need to steal glances at Jia Jia.
The two will be put together, at least temporarily. They will be separated again when the mating period ends. Pandas are solitary animals by nature.