The 25-year-old sealed victory with a birdie on the par-5 18th at Royal Golf Dar Es Salam, finishing at 19-under and one shot clear of Watson after a tense final-round battle.

Kho, who made history by winning Hong Kong's first golf gold medal at the Hangzhou Asian Games in 2023, was superb all week on one of the Asian Tour's toughest tests, making just three bogeys across 72 holes.

Starting the final round tied for the lead, Kho raced clear with five birdies in his opening 10 holes to move four-under for the day. However, he was unable to find the decisive birdie that would create separation until the final hole.

Overnight leader Watson (70) drew level with Kho after a bunker birdie at the 17th, but the two-time Masters champion pulled his second shot on the 18th towards the LED screen and failed to save par, opening the door for Kho to secure victory.

The win earns Kho US$360,000 and lifts him to No.2 on the Asian Tour Order of Merit behind Australia's Travis Smyth, while also moving him to No.3 in The International Series Rankings.

It was a fitting response after the disappointment of losing in a playoff on the Japan Golf Tour just a week earlier.

"It's been a long year. It's had a lot of hardships and a bit of adversity. Through it all, I think I really grew as a person. I realised the work I put in surpasses any results," said Kho.

"Golf's a funny game. It really gives and takes, and I was very fortunate the last couple of weeks to be on the good side of it. I just have a lot of gratitude towards the game itself, the people around me, the people who make it possible."

The victory carried extra significance given Kho grew up idolising Watson.

"I was so pumped up on the golf course. I think playing with Bubba is like a dream come true. I remember watching him win two Masters when I was a little kid. For me to be going neck and neck with him for five hours today, the child inside me was jumping up and down," said Kho, who admitted victory only entered his mind after making birdie on the final hole.

"I didn't even think of winning until the last putt dropped! Golf is a really psychological game, and I think one thing that I continue to learn is that a lot of thoughts are going to go through your mind, whether they're good or bad, and it's my ability to be able to play freely and play to my instincts through those thoughts. That is what really made the difference for me.

"My last four-footer winning putt felt like a truly impossible putt. I did not know how it was going to break. The putt felt so long, and the hole felt so small, but there was a part of me that knew I had the ability to be free in that situation. That's what really helped me hit a good putt. It's a real mental battle out there, but I'm really glad to get that done."

Thailand's Suteepat Prateeptienchai (69) and Jazz Janewattananond (72) shared third place at 15-under. Suteepat bogeyed his final two holes, while Janewattananond dropped a shot at the last.

Australian Elvis Smylie continued his strong season, producing six birdies across his final seven holes for a closing 66 to finish tied fifth at 14-under alongside South Africa's Ian Snyman (67), Zimbabwe's Kieran Vincent (67) and Japan's Takanori Konishi (71).

Despite falling one shot short of a first victory since the 2018 Travelers Championship, Watson was upbeat about both his own performance and Kho's breakthrough win.

"I played great. I played as well as I could. Taichi beat me by one, and it was great to see a young player like that perform so well. It shows the game of golf is in a great spot, and it was fun to watch how well he played today," said the 47-year-old RangeGoats GC captain.

"He actually told me after the round that he used to watch me growing up, so that was pretty special.

"I told him he's a great player. If he keeps doing what he's doing, he's going to have a great career."

Janewattananond, the inaugural International Series Morocco champion in 2022, was left frustrated by a closing bogey that cost him outright third.

"I made too many mistakes today. Last hole stung a little bit, because I lost a lot of money on the last hole, but it is what it is, you know. Sometimes you win, sometimes you lose," said the former Asian Tour Order of Merit winner.

"I think the game is close. The momentum needs to be better. Swing and everything else is on the right track. So, excited to see what I have for the rest of the year. My goal for this year is to win more than once, because it's close. I can feel it. I just need a little bit more momentum. I need to learn how to win again, and then once I get it done, I think the second one should be easier."

The International Series Morocco concluded a three-week stretch for the Asian Tour in Morocco. The circuit returns in September for the Yeangder Taiwan Open.