Watson reached a career-high 38 in the world rankings in 2015Heather Watson reached her first quarter-final in 18 months with a three-set win over South Korea's Han Na-lae at the Thailand Open.
The British number six triumphed 6-3 5-7 6-4 in a match lasting two hours 33 minutes.
Watson, 30, will face Chinese seventh seed Wang Xinyu in Hua Hin on Friday.
Victory over Han means Watson - currently 160th in WTA rankings - will return to the top 150.
She led by a set and a double break before Han fought back to force a deciding set.
Watson broke again in the third set, but threw her racquet in frustration when Han again levelled at 4-4, before wrapping up the win with a drop shot on her second match point.
Watson lost in the semi-finals of the Birmingham Classic in June 2021, the last time she reached at least the last eight of a WTA Tour event.
