Rebounding from their crushing 491-run loss in the first Test, Pakistan's top order batsmen showed greater resolve and were more comfortable on the lower, slower Melbourne Cricket Ground pitch than last week's bouncy conditions in Perth.
Leading his country for only the second time in his 55-Test career, Youhana scored his 12th century off 117 balls, finishing on 111 when he was stumped down the leg side off the bowling of Shane Warne at 286 for four.
Youhana batted for three and one-quarter hours, hitting 11 fours and four sixes from 134 balls for his first century against Australia.
The 30-year-old right-hander dominated a 192-run stand with Younis Khan (87) after Pakistan had lost three wickets for nine runs to be struggling at 94 for three shortly after lunch.
The partnership was a record fourth-wicket stand for Pakistan against Australia, beating the 177 by Saeed Anwar and Youhana in Brisbane five years ago.
After the tourists won the toss and elected to bat, 20-year-old opener Salman Butt scored his maiden half-century in his third Test, hitting 70 from 99 balls.
Paceman Jason Gillespie (3-72) took two wickets in successive overs late in the day's play.
EARLY CHANCE
Butt, who hit 10 fours, was dropped by Matthew Hayden at gully in the first over of the morning bowled by Glenn McGrath.
Imran Farhat (20) put on 85 for the first wicket with Butt, 13 runs more than Pakistan's second innings total in the first Test.
Farhat was caught at second slip by Australia captain Ricky Ponting off the bowling of Michael Kasprowicz.
Pakistan were 93 for one at lunch after winning the toss and batting.
However, Australia hit back immediately when Gillespie had Yasir Hameed lbw for two in the first over after lunch.
Butt fell in the next over when he failed to stretch out attempting a second run and was beaten by a strong throw from third man by Michael Clarke at 94 for three.
Youhana was also dropped by Hayden, who spilled a head-high chance at gully when the batsman was on 43.
After Warne (1-100) dismissed Youhana at 286 for four, Younis fell 12 runs later, caught behind by Adam Gilchrist after edging a leg-cutter from Gillespie.
Younis hit 11 fours and batted for thee-and-a-half hours.
Gillespie struck again in his next over when Ponting took a juggling catch at second slip to remove Shoaib Malik for six at 301 for six.
Injured captain Inzamam-ul-Haq and paceman Mohammad Khalil were replaced in the Pakistan side by Hameed and Malik.
Australia named an unchanged side after opening batsman Justin Langer passed a fitness test on his injured back.
