Laura Robson, at 18 the youngest woman in the top 100, became the first British woman since Jo Durie in 1990 to reach a WTA Tour final when she lost to Hsieh Su-Wei in Guangzhou three weeks ago.
Watson, two years Robson’s senior, showed she can match her fellow Briton with a fine win over the Japanese in just over two hours.
After a tight first set in which they shared a break apiece, Watson dominated the tie-break, winning five points in a row after losing her initial mini-break.
Both women had served well in the first set, which was not the case as they found themselves unable to hold serve for much of the second, trading breaks until Doi finally held at 3-2.
Both women appeared to settle as Watson in particular found her service rhythm, landing upwards of 80 per cent of her first serves.
And Watson broke for a third time in the set to give herself a chance to serve for the match, only to freeze and allow Doi to break back far too easily.
She got a second bite at the cherry though, breaking once more and this time holding comfortably to move into a WTA final for the first time in her nascent career.
Watson will face another player making her first WTA final appearance after Chang Kai-Chen upset Samatha Stosur 6-4 4-6 7-6(3) in the second semi-final.
The Taiwanese world number 134 had never previously gone beyond a quarter-final in a main tour event, giving Watson a fantastic chance of becoming Britain's first WTA Tour singles title winner since Sara Gomer in 1988.