Noted Japanese matchmaker Joe Koizumi, in his report posted on fightnews.com, described the bout as "such a close affair that there were arguments for and against the verdict."
Judges Anek Hongtongkam of Thailand scored 114-113 and Ukrid Sarasas of Japan saw the bout at 115-113, both for Yamaguchi, while Alex Vidal of the Philippines penned it 115-112 for Santillan, thus, the split verdict.
Koizumi believed Yamaguchi's relentless attack in the last round was the turning point of his victory as the score was tied at 104-104 on the scorecard of Hongtongkam after the 11th round.
"Yamaguchi, in the final canto, displayed his last surge and gained an important point on the neutral referee Hongtongkam¡'s tally (which had been 104-104 after the eleventh)," said Koizumi.
Koizumi said that Yamaguchi started strong and took the first two rounds. Santillan sustained a cut over his left eyebrow in round two, but Yamaguchi was also bleeding from a gast over his left optic from the third round on.
Santilan found his range and rhythm to be in command in middle rounds and in the 11th, the Filipino southpaw floored Yamaguchi with a solid left shot. Santillan furiously followed it up, with Yamaguchi barely withstanding the fiery assault.
Yamaguchi fought aggressively in the last round on the way to eke out a close victory, his 15th straight in 16 fights that include a draw and 11 wins by knockout. The WBC ranked No.11 welterweight Santillan dropped to 22-3-1 with 16KOs.
This is the third time Santillan lost the OPBF 147-pound crown he seized by conquering Korean Suk-Hyun Yun via split decision on January 26, 2001 at the Waterfront-Cebu City Hotel.
After two successful defenses, the 28-year-old Santillan was dethroned by Japanese Hiroshi Watanabe by split verdict in their rematch on October 14, 2002 at the Park Arena in Komaki, Japan.
Santillan regained his throne by defeating Watanabe also by split decision in their third encounter, but surrendered it for the second time after being knocked out by Japanese Kazuhiko Hidaka in the fourth round on March 19 last year in Tokyo.
Santillan reclaimed the OPBF championship by scoring a vengeful eight-round stoppage of Hidaka in their return bout on December 3 last year, only to crumble once more at the hands of Yamaguchi.