After chalking up about 15 endurance races a year for the past six years, Mr Norhalim Nordin was ready for a change of scenery.
Together with five buddies, the 29-year-old supply chain coordinator signed up for the Nathan Singapore City Race in March, his first free-form race.
The 45km event has no fixed route. Instead, participants have to clear five checkpoints scattered across the island in the shortest time on foot, using a map.
"Free-form races get you to explore places you've rarely been to, instead of the similar routes where half and full marathons are always held," he says.
Without a set route or signs to guide them, participants have to plan the quickest way to get to the checkpoints pre-marked on a physical map, which may sometimes come with clues.
These races are usually competitive and prizes, which include cash or products, are awarded to those who finish first.
Mr Norhalim, whose group took eight hours to complete the race and came in 10th out of 137 teams, likens the experience to a treasure hunt with time pressure.
Others might dub it Amazing Race, after the popular American reality TV series.
The technical term for this activity is "orienteering".
It involves navigating an unknown area to look for clues or checkpoints. Participants use a map and sometimes a compass, to find their way around on foot.
This may be familiar ground for those who have served in the army or were scouts or girl guides in school, but the activity is also becoming popular among the masses.
At least six races organised here this year contain some element of orienteering. Two of them - Nathan Singapore City Race and Ubin Wayfinder - revolve solely around navigational skills and made their debut last year.
The Nathan Singapore City Race, which drew about 3,500 participants last year, saw 4,000 people this year. Organised by sports event organiser Pink Apple, the race has multiple categories with distances that range from 5 to 45km.
Each route features five checkpoints which the organiser describes as "hidden treasures", such as a portcullis (a vertical iron gate) at Labrador Park and Sri Thendayuthapani Temple in Tank Road.
Ubin Wayfinder, a similar race on a smaller scale, takes place tomorrow at Pulau Ubin. Held by outdoor sports events company Osportz, it will flag off with about 160 racers, up from about 120 last year, and will cover 8km and 10 checkpoints.
Both races can be done solo or in groups.
While conventional races test stamina, orienteering races require map-reading skills, a good sense of direction and, in the case of group challenges, communication skills.
The level of technical skill can differ from race to race. For example, maps might suffice for some but others might require a compass.
The lack of signage, recognisable landmarks and spotty cellphone networks add to the level of difficulty for the participants of Ubin Wayfinder.
Checkpoints for the race have names such as Knoll- North/Fallen Tree, Quarry View or simply Trail.
For those used to relying on GPS, having to read physical maps and compasses can be a big hurdle.
Teacher Ana Wong says with a laugh: "It will be challenging. I'm totally new to Pulau Ubin. Maybe I should read up on how to use the compass."