Alexander Volkanovski says he lost nearly two stone (25.5lb) in 11 days to make weight for his rematch against Islam Makhachev at UFC 294 on Saturday.
Volkanovski weighed in at 154.5lb, while Russian champion Makhachev, 31, was at the lightweight limit of 155lb.
"Obviously I was holding a lot of water," Volkanovski told TNT Sports.
"I was on a little holiday with the family, and I was enjoying it. I was enjoying the sun, had a couple of drinks and eating very well.
"But it's funny, I had a day head-start before I got the call [to fight] because I saw that weight and I was like 'oh wow, I'm not happy with that'. So when I did get the phone call, I was [already] a kilo or two lighter.
"You have to move some [weight] but my body just knows, I flick a switch and the weight went off and my body just kicked into gear."
Featherweight title holder Volkanovski is attempting to become just the fifth simultaneous double champion in UFC history, following Conor McGregor, Daniel Cormier, Amanda Nunes and Henry Cejudo.
Makhachev nullified Volkanovski's first attempt at winning his second belt in February, edging a close decision.
If roles were reversed, this fight is not happening - VolkanovskiI heard my leg crack seven or eight times - Muhammad MokaevChimaev and Usman make weight for middleweight clash
Chimaev (pictured) and Usman have moved up to middleweight for their bout in Abu DhabiIn the co-main event, Nigeria's Kamaru Usman weighed in at 184.5lb for his middleweight bout against Chechen-born Khamzat Chivaev, who was 185.5lb.
Like Volkanovski, Usman, who has fought the majority of his career at welterweight, was a late replacement for the fight after Paulo Costa pulled out injured.
Chimaev is also moving up in weight after missing the 170lb welterweight limit for his bout against Nate Diaz at UFC 279 last September.
The misdemeanour caused a reshuffle of the card, with Chimaev fighting Kevin Holland at a catchweight of 180lb, and Diaz taking on Tony Ferguson in the main event.
Elsewhere, Britain's Muhammad Mokaev and Nathaniel Wood made weight for their bouts on the undercard.
Mokaev, 23, is taking on Tim Elliott at flyweight, while Wood, 30, faces Muhammad Naimov at featherweight.