Samuels was unbeaten on 50 while his partner Darren Bravo was batting on 30 as West Indies still trail Pakistan's first innings total of 579-3 declared by 441 runs with eight wickets intact.
The West Indies first target will be to avoid follow-on by reaching 380.
Samuels and Bravo have added 69 for the third wicket after West Indies -- resuming at 69-1 -- lost opener Kraig Brathwaite leg-before to leg-spinner Yasir Shah for 32 in the second over of the day.
Samuels took a single off spinner Mohammad Nawaz to reach his 24th Test fifty, having so far hit eight boundaries.
Bravo was more cautious, having hit three boundaries and a six.
Shah has so far taken both the wickets for 42 in his 18 overs.
Pakistan's big total was built around opener Azhar Ali's unbeaten 302 which is the fourth triple century for his country.
The remaining two Tests will be played in Abu Dhabi (October 21-25) and Sharjah (October 30-November 3).
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