The below is a beautiful display of Muay Thai. Both Buakaw and Petnamek throw beautifully and utilize their technique in rather different strategies to impose their will on the fight.
This bout, in addition to being a fun to watch display of Muay Thai, also serves to highlight a difference in scoring between Muay Thai and kickboxing or Western boxing. Namely, that the judges weigh the last two rounds heavily over the first three. This makes sense on paper, but for those of us who are used to seeing each round weighed equally, it seems strange on viewing.
Of the fighters, Buakaw is undoubtedly more famous, due to his successes in K-1. At this point, he was still fighting in the 135 - 140 lb (61.5 - 63.5 kg) range. It would be two years before his debut in that organization. He now fights at 154 lb (70 kg), which is also his walk-around weight.
Like Buakaw, Petnamek Sor Siriwat was a well-regarded fighter on the Muay Thai circuits during this time. He too would move up in weight. I believe the most recent footage of him on Youtube showed him participating in Muay Thai vs San Da in 2005.
Buakaw fights out of the blue corner in this bout, Petnamek, southpaw, fights out of the red corner.
Thanks to LOOPNOUB and his awesome Youtube channel for this video. After his ten minute wai kru (heavily influenced by Namsaknoi's), Buakaw clearly dominates the first two and a half rounds. He moves well around Petnamek and counters beautifully, displaying well-rounded skills. He even seems to have Petnamek in trouble for a bit in the second.
Petnamek, for his part, weathers the attacks well and, importantly, constantly moves into Buakaw. He doesn't get through as much as Buakaw until the fourth and fifth rounds, but the decision came to him because he showed himself unfazed by earlier damage and gradually ramped up the pressure until he was backing up a clearly tired Buakaw in rounds four and five.
To someone used to scoring by points and by round, instead of the 'marathon' style of scoring used in Muay Thai, the decision might look strange because Buakaw clearly took rounds one and two, likely three, and landed clean shots and defended well in the last two rounds. However, take into account that the first two rounds are usually scored as a draw or with a half-point difference and Petnamek is quite clearly the winner. It's true that Buakaw's constant output closes up the fight, but the last stages of the fight largely consist of Buakaw trying to shrug off a Petnamek pressing the issue with clinch attacks and other forward pressure.
Losing the fight on points is one possible consequence of expending too much energy early. A fighter might also be KOed, as Buakaw was against Sato. The reason he gave in this upset was that he'd used too much energy in the early rounds trying to KO Sato, which might, interestingly, have been encouraged by K-1's own scoring system.
I mention this aspect of the fight because I've been asked why Muay Thai fighters start the first rounds slow. Some people in the Thai Muay Thai scene take issue with how fights are currently scored, but you'll have to find someone with more knowledge than I to discuss that in more than the vaguest strokes.