There were no less than seven breaks of serve in a sporadic opening set, before Latvian Ostapenko, the world number seven, forced a decider following a tie-break after Williams had failed to convert a match point chance.
Karolina Pliskova kicked off play for the White Group at the BNP Paribas WTA Finals Singapore presented by SC Global in emphatic fashion on Sunday, surrendering just four games to defeat Venus Williams, 6-2, 6-2.
Earlier, Williams revived her WTA Finals campaign with an epic three-hour, three-set victory over Ostapenko.
Former world No. 1 Williams has 49 singles titles including seven Grand Slams, amassed over 19 years starting from 1998, while 20-year-old Ostapenko's two career titles were won only this year. I lost the last match against her.
"I had to work for every point, she came up with shots you can not prepare for". "It's not easy to lose the first match, and both of us were in the same situation".
"Honestly, it was all credit to her, she came up with a lot of shots that you can't prepare for and that's why she's here in this tournament - she has a lot of talent and I was just happy to win that last point".
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Ostapenko seized the initiative in the opening game before a double fault immediately restored parity, something she repeated after fighting from 40-0 down to break in game five. At 4-5 down and serving to stay in the match, a spectator led a chant of "Let's go, Venus", but the Latvian had other ideas.
Muguruza, though, saved three match points to hold serve and then broke back. Momentum on her side, Ostapenko won the tie-break 7-3 to force a third set.
Both players, though, continued to struggle to hold serve in set three, which the American number five seed eventually secured after one hour and 10 minutes.
Even though the match against Ostapenko will be a dead rubber, Pliskova said she wanted to continue her rich vein of form. "When I had to serve, I didn't really serve (well)".
If the first encounter of the day was a marathon, the second was a sprint as Czech third seed Karolina Pliskova swatted aside Spaniard Muguruza 6-2 6-2 in just 62 minutes to advance to the last four as pool victor with two victories from as many ties.
Williams, who made the final of both the Australian Open and Wimbledon, will face Muguruza on Thursday looking to keep alive hopes of securing a place in the knockout stage.