Younus 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 leave captain Misbahul Haq and Asad Shafiq to continue the workmanlike effort heading into the fourth day.
Younus Khan becomes first Pakistani to reach 10,000 Test runs
Having confirmed this series as the last before he retires from international cricket, 39-year-old Younus 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, Younus 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.
Twitter reacts to Younus’ 10,000 runs
"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," said Khan 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."
Hailed as a potential successor to Younus as the permanent occupant of the number three batting position, Azam enjoyed a moment of luck just after lunch when West Indies captain Jason Holder failed to hold on to a caught-and-bowled chance with the batsman on 12.
Yet just as it looked as if their partnership would continue until the close of play, Gabriel broke through.
Younus mistimed an off-drive to give to be caught at extra-cover and become the fast bowler's 50th Test wicket. Then in his next over a tentative Azam played on to spark further celebrations among the West Indian players.
Twitter reacts to Younus’ 10,000 runs
Earlier, Holder finished unbeaten on 57 after the delayed start to another rain-affected day as Mohammad Amir completed his best Test innings figures to wrap up the home side's innings after they resumed at the overnight position of 278 for nine.
He bowled last man Gabriel to finish with six for 44, an effort which improved on his previous best of six for 84 against England at Lord's in 2010.
Play started 90 minutes later than scheduled due to the continuing effects of heavy, torrential rain on a damp outfield which reduced play to just 55 minutes on the second day.
Pakistan started their reply brightly with Azhar Ali looking ominous before he chased a wide delivery from Alzarri Joseph to give a catch to wicketkeeper Shane Dowrich.
Ahmed Shehzad, playing his first Test for almost two years, also opened brightly with a flurry of shots, including five boundaries in 31 only to be trapped leg-before on the back foot by Holder.
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