Kim Kardashian's keeping Lewis Hamilton loose before his race in Singapore this weekend ... making him crack a smile in her cheeky pink outfit.

The reality star and mogul is hanging out in the Southeast Asian country this weekend ... and she donned an impressive dress with a high neckline and no sleeves.

The garment's a bit see-through around the waist ... giving fans a look at some of her buxom backside.

Kim and Lewis saw each other briefly ... bringing a grin to his face -- though he's obviously focused on the Singapore Grand Prix. He finished with the third-highest qualifying time, so he'll start high up in the field for Sunday's race.

Don't worry about Kim, though ... she's busy hobnobbing with celebs like Romeo Beckham and looking hot while doing so.

The race in Singapore kicks off at 5 AM PT Sunday ... and we're sure Kim and Lewis will be living it up if he pulls out the win!