Younis moved on from a painfully slow start to get to 58 while Babar Azam stroked 72 in dominating a 131-run third- wicket partnership with the former captain to give the tourists the ascendancy into the final session of the day.
However, fast bowler Shannon Gabriel dismissed both in the space of three deliveries to limit Pakistan's progress and leaving captain Misbah-ul-Haq and Asad Shafiq to continue the workmanlike effort heading into the fourth day.
Having confirmed this series as the last before he retires from international cricket, 39-year-old Younis played with exaggerated care at the start of his innings. In contrast, Azam displayed much more aggression and purpose although he once again fell short of a maiden Test century.
Possibly mindful of his proximity to the landmark when he started his innings 23 runs away from the cherished goal, Younis took 47 minutes to get off the mark while Azam attempted to keep the scoreboard ticking over at the other end.
Playing his first Test in the Caribbean after missing the 2011 tour due to the death of his brother, the former captain eventually started to creep towards his 10,000th run, triggering the celebrations when he swept a delivery from off-spinner Roston Chase in the first over after tea to the backward square-leg boundary to become just the 13th player overall -- and the oldest -- in the history of the game to scale that summit.
"I thought about bowing out of international cricket three years ago but one of the motivating factors for me to stay was to get to ten thousand runs," Khan said at the end of the day.
"I am finished after the series, no matter how many runs I score, but I just hope I have been able to make a useful contribution as a player and also help the younger ones to do as well as I have, or even better."