Trailing by a set and facing a 4-1 deficit in the second set against Collins, Sakkari completed a shocking turnaround to beat the American 3-6 7-5 6-2 in the semi-final.
The 23-year-old will face Romanian Mihaela Buzarnescu who reached the final by beating fourth seed Elise Mertens 4-6 6-3 6-1.
Sakkari persevered past Williams on Friday despite trailing in both sets, and she again displayed the knack for a comeback. She won four straight games in the second set to put pressure on Collins, and ultimately evened the match with a forehand winner.
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Collins, also looking to make her first WTA final, never recovered.
Meanwhile, Buzarnescu will get another shot to break through for her first career WTA singles title after rallying back against Mertens.
In a rematch of Hobart International final in January, Buzarnescu fought back from a set down to get revenge and set up the biggest match of her career.
"I’m so pleased and happy to be in the final, the third final of my career," Buzarnescu told reporters.
Even being able to play semi-finals at more WTA tournaments was a great result for me. I really need to look at that and use it to stay strong and positive for the future.
Coming into her semi-final match, Buzarnescu was facing an opponent in Mertens who had only been broken on service twice during the tournament. After a back and forth opening set, Buzarnescu reeled off four games in a row during the second set to take control and never looked back.
Mertens called for a trainer in the final set due to a right forearm injury, and Buzarnescu won six games in a row to closer her out. The victory puts Buzarnescu into the first Premier-level final of her career, and will also see her rise to a new career-high ranking of world number 21 when the new WTA rankings are released on Monday.
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