Joshi, who was sixth overnight, slipped in the early part of the day as he was three-over with two holes to play. He then birdied the last two holes to climb up the leaderboard.
Most of the other Indians also registered a drop in final placing as Rashid Khan (76) ended 35th, down from 21st, while Abhijit Chadha and Angad Cheema, both carded 77 each to finish tied 55th.
Also at the same place was Chiragh Kumar (72), the only one to move up to tied 55th, from overnight 63rd.
The former world number one's bid to claim his hat-trick of titles at the USD 750,000 Asian Tour event was secured when he birdied the play-off 18th hole as Chapchai, a four-time Asian Tour winner, could only make par.
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"There are a lot of good players on the Asian Tour now and the standard is getting higher on this Tour and it's harder to come over here and win, especially today.
The Englishman appeared to be cruising towards an emphatic victory until two successive bogeys on 16 and 17 blew the title race wide open.
He had the opportunity to win with a birdie at the last but misread the green and was forced to settle for a par and a four-day total of seven-under-par 281. But after returning to the 18th hole for extra-time, Westwood made no mistake from the exact spot again.