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Standing 2.11m tall, Karlovic's huge serves and unorthodox style caused problems for some of the sport's biggest names and he won two of his three meetings with world No, 1 Novak Djokovic.
The 22-year-old fourth seed is the first Italian to win the Melbourne Park title.
Bopanna also became only the third Indian to win a men's doubles Grand Slam title.
The Belarusian second seed is the first woman to retain the Melbourne Park title since compatriot Victoria Azarenka in 2013.
Sinner broke for a 3-1 lead in the fourth set and ran away with the match to snap the Serb's 33-match winning run at Melbourne Park and end his quest for a record 25th major.
Sinner's victory snapped Djokovic's winning streak of 33 matches at the Australian Open going back to 2018 and was the world No 1's first loss in 11 Melbourne semifinals.
Sabelnka beat fourth seed Coco Gauff in a thriller on while Zheng got the better of Ukrainian qualifier Dayana Yastremska.
Bopanna and his Australian partner Matthew Ebden prevailied in a tense semifinal against Tomas Machac and Zhang Zhizhen.
Gauff set up a semifinal clash with Sabalenka in a rematch of last year's US Open final.
The top remaining seed after Saturday's early exit for world No. 1 Iga Swiatek, Sabalenka's biggest enemy has often been her own emotions but she continued to keep them strictly under control to reach the last eight without dropping a set.