No lies detected here -- 'cause recently retired tennis superstar Caroline Wozniacki did an impromptu photo shoot while on a hike in France ... and all she needed was a tiny yellow bikini!!

The 30-year-old and her ex-NBA player husband, David Lee, are enjoying a "Staycation" in Saint-Jean-Cap-Ferrat, France ... just a quick 30-minute drive from one of their homes in Monte-Carlo, Monaco.

Wozniacki and Lee recently had their 1-year wedding anniversary in June and celebrated her birthday last month ... so there's been a LOT to celebrate!!

Of course, Wozniacki decided to hang up her racket back in January ... but there's no retirement bod here -- her abs are as hard as the rocks she's laying on!!

The former #1-ranked tennis player has been a fan of bikinis for years -- see here, here and here -- and even posed in the Sports Illustrated Swimsuit issue back in 2015.

Nice to see nothing has changed in retirement life!!