In their second essay, UP were 14 for no loss at stumps with the openers opting to see off Mohd Asif and Sarfaraz Ahmed towards the end as the ball moved around the seam.
The day, however, belonged to Nazir, who turned the heat back on the UP bowlers who were looking to run through the Sialkot batting.
The opener, who was retired hurt after playing just two balls, returned to the field when Sialkot were struggling at 96 for six.
Nazir chose to attack instead of hanging around and ran amok to complete his century in just 96 balls. Such was his dominance that he hit five towering sixes and 16 fours in his 113-ball knock.
Nazir, who did bulk of the scoring, got a suitable partner in Tahir Mughal, who dropped anchor at the other end and scored 29 off 63 balls rotating the strike well.
Together the duo scored crucial 122 for the seveth wicket before Piyush Chawla castled Mughal with a perfectly pitched googly.
Chawla, who was taken for some runs by Nazir before, cameback in style and picked up Mohammad Asif on the very next ball, again a googly, as Sialkot scorecard read 218 for eight.
With Sarfaraz Ahmed dismissed for four, Nazir stepped on the gas and went after every delivery eventually holing out one from RP Singh to Shukla.
Earlier, the UP bowlers bowled a tight leash giving little room to rival batmen. Shahid Yousuf tried to wield his willow and the trick seemed to work with the batsman reaching to 24 in no time.
Just when he looked in control, Shahid edged RP to wicketkeeper and the floodgates were opened.
Shoaib Malik tried to put some resistance in the middle but was undone by Shrivastva for 24 (59 balls).
In the morning, UP resumed at 269 for seven and had an inauspicious start with Sarfraz Ahmed getting Amir Khan on the second ball of the day. However, the tail wagged as Rizwan Shamshad held one end. Shamshad topscored with 84 (193 balls, 4x4) was undone by Abdur Rehman but not before putting his team to a respectable 316.