After winning two of the first three frames, Wattana reached a significant turning point in the fourth frame when McManus, with a break of 53, led 64-22. However, after missing a long red, the Scot saw Wattana clear to the pink before taking the frame
65-64 on the black. The next frame proved just as crucial, Wattana again winning on the black with a clearance of 64.
From there, the Thai stretched his advantage to 7-1 and looked in typically commanding form. McManus briefly held up Wattana's run, winning the ninth and 10th frames. However, Wattana responded with breaks of 49 and 85 to seal his 9-3 victory in five minutes short of three hours.
Only Jimmy White, the newly crowned United Kingdom champion, has gathered more ranking points this season than McManus. The 21-year-old Glaswegian is not only the Scottish No 1 after successive victories over Stephen Hendry, but also No 2 in the world. However, his predicament confirmed his pre- match fears of Wattana.
'I believe James is one of the top six players in the world, but I can't say that about myself. I like to go into matches believing I'm the underdog because that helps me relax,' McManus said before appearing in his fifth semi-final of the season.
'I think the same approach could help Hendry because he seems to be putting himself under a lot of pressure.'
Wattana, provisionally ranked fifth, eliminated John Parrott, the former world champion, in the quarter-finals. He is in the kind of form that could trouble Davis, even after Davis's record-breaking 9-4 win over Martin Clark in Thursday's first semi-final.
Davis, world champion six times, made total clearances of 141, 123 and 135 in the opening session - the first occasion any player had achieved the feat.
'I'm striking the ball well and my safety is good, so I haven't got too much to complain about,' Davis said. 'Whoever I play in the final, it's going to be tough.'
COALITE WORLD MATCHPLAY (Doncaster) Semi-final: J Wattana (Thai) bt A McManus (Sco) 9-3.