This is after 90-year that a batsman has scored three back to back centuries against Australia. Before Younis, English player Herbert Sutcliffe scored three centuries against Australia in the year 1924.
Mudassar Nazar, Zaheer Abbas and Muhammad Yousaf were other three Pakistani players to made three back to back centuries in test innings.
Pakistan, who won the toss and elected to bat on a flat Sheikh Zayed Stadium pitch, were 304 for two at close on the opening day of the second Test.
Younis Khan ended the day with unbeaten 111 while Azhar Ali remained 101 not out.
Pakistan lost openers Ahmed Shehzad (35) and Mohammad Hafeez (45) in the two sessions.
Pakistan, leading the two-match series 1-0, remained unchanged from the side which won the first Test by a big 221-run margin in Dubai.
Australia, seeking to avoid their first series defeat against Pakistan since 1994, made two changes with Glenn Maxwell and Mitchell Starc replacing Steve O’Keefe and Alex Doolan.