Williams survives Ostapenko ordeal
Venus Williams came through a marathon encounter against Jelena Ostapenko to record her first victory at this year's WTA Finals, triumphing 7-5, 6-7(3), 7-5 in three hours and 13 minutes.
The pair's only previous meeting came in the Wimbledon quarter-finals this year, with Williams winning 6-3, 7-5, but this was a far tighter affair.
Each of the three sets could have gone either way, with Williams taking the opener after recovering from 5-3 down to win four consecutive games.
In the second, Williams had a number of opportunities to finish Ostapenko off - including a match point at 6-5 - but the closer she got to victory, the better the Latvian played. A tie-break was needed to separate the duo, but Williams faltered, allowing Ostapenko to force a decider.
There, the momentum continually switched between the two players, with neither able to fully capitalise on the openings created. A run of six consecutive breaks ended at 4-4, but Ostapenko's serve remained vulnerable, and Williams grabbed the decisive break at 5-5 before serving it out.
While Ostapenko did hit 48 winners, her serving was her undoing, with Williams constantly pushing for a break - the veteran American generated 26 break points in all, and while she only converted 11, it was enough to get over the line.
“I was just a little bit tight in some moments,” said Ostapenko. “When I had to serve, I didn't really serve."
“[Venus] was playing good today. She was serving well. It was hard to break her, but I think I was returning pretty well today, better than serving. We had a very long match and some good rallies."
Pliskova qualifies in style
Karolina Pliskova advanced to the semi-finals of the WTA Finals for the first time in her career thanks to a 6-2, 6-2 victory over Garbiñe Muguruza.
The Czech laid down an early marker in Singapore by beating Williams 6-2, 6-2 in her opening appearance, and she only expanded on that theme here, blitzing past Muguruza to further stake her title credentials.
At no stage in the match did Pliskova trail her opponent, twice racing out to 3-0 leads that would prove unassailable. Muguruza was understandably unable to get a handle on Pliskova's big serving, and though she did rally to save three match points at 5-0 down and then claim her first break of the match, she was promptly broken to love herself, handing Pliskova her second victory of the week.
"I know she can play very good tennis," said Pliskova.
"I have had couple of matches where we played amazing tennis together, and all the matches were pretty close except I think two, so this one is one of them.
"I remember the last one I lost in Cincinnati, so I just really wanted to be ready from first ball and not give her those chances.
"That's why I started so well. I was up both sets three-love, which was important for me to feel that confident from the head-to-head matches."
The result sees Pliskova qualify as winner of the White Group, and she will end the round robin stage with a dead rubber against the now-eliminated Ostapenko. Second place, however, will come down to a rematch of the Wimbledon final between Muguruza and Williams.