World number three Pliskova won her opening two White Group ties against Venus Williams and Garbine Muguruza to book a semi-final berth with a match to spare but the Czech was out-gunned by an inspired Ostapenko at the Singapore Indoor Stadium.
Ostapenko had battled hard in her opening matches to no avail, especially in a gruelling three-set loss to Williams, but she served brilliantly against Pliskova and found the corners with her booming forehand to end her season with a win.
"Yes, I lost my earlier two matches but it's my first time here and I am just happy to finish my season here, in the top eight, especially with a win at the end," Ostapenko said in a courtside interview.
Spanish world number two Muguruza and American veteran Williams will meet later on Thursday, with the winner of that contest advancing to the semi-finals along with Pliskova.
The 20-year-old Ostapenko rode her high-risk strategy to a stunning French Open triumph in June and after pushing her luck a little too much in her opening defeats, the Latvian was back to her best as she stormed to victory in 66 minutes.
The pair traded a couple of early breaks in a slow start to the match before Ostapenko put her foot on the accelerator and raced away when she took a pivotal seventh game by forcing Pliskova into a forehand error on her fifth break point.
Another break handed the Latvian the opening set in 43 minutes as the big-serving Pliskova continued to struggle with her most potent weapon in the second and a double-fault handed Ostapenko an early break for a 2-0 lead.
The Czech finally held to halt Ostapenko's run of games at seven to make it 3-1 in the second, but the Latvian maintained her high standard of play to stay ahead of a flat-footed Pliskova, and another break took her to the brink of victory.
Pliskova looked as if she already had her mind on Saturday's semi-finals and stood rooted to the spot as an Ostapenko ace brought up three match points and the Latvian only needed one of them when her opponent sent a forehand wide.
"She's a great player who has had a great season but I had nothing to lose so just went for it," Ostapenko added. "It's been an amazing year for me, winning the French (Open) and reaching the WTA Finals, so I am really happy with how it went."
(Editing by Amlan Chakraborty)
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