Venus Williams beat Roberta Vinci in a closely fought three set match, settling a score on behalf of her sister Serena, to reach the Wuhan Open finals.
The seven-time Grand Slam-winner was a match point down to the Italian at one stage, but eventually saw her off in a final set tie break.
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"I mean (there were) a lot of ups and downs. I hope that on a day when I'm playing how I'd like it's a more straightforward match. She's definitely tricky. She finds a way to hang in there," said Williams.
Williams looked comfortable going into the third set, securing a 4-1 lead over Vinci.
But the shock US Open finalist pushed back, evening the score before finally falling to the American veteran 5-7, 6-2, 7-6 (7/4).
"I'm of course really sad because I lost. But if I think all the tournament (it) is okay. I played incredible, won four matches here, (a) semi final, 7-6 in the third against Venus," said a stoic Vinci.
It was Vinci's second consecutive semi-final meet against a member of the Williams family, after downing world number one Serena in the US Open, an upset that denied her a calendar-year Grand Slam and a record-equalling 22nd major title.
"If I could I would give this win to Serena at the US Open, but it doesn't work like that in tennis," Williams said on court after the match.
Williams — who with 46 career titles has more wins to her name than the other semi-finalists combined — will face Wimbledon finalist Garbine Muguruza on Saturday.
Muguruza downed Germany's Angelique Kerber in two sets 6-4, 7-6 (7/5), despite falling during the second set tie break and injuring her ankle.
"I knew (it) was going to be a very physical match. You know, (I'm) happy to be able to win in two sets because I never win against her in two sets," said Muguruza.
The 21-year-old Spaniard and world number eight entered the semi-finals as the highest ranked player, after retirements and shock defeats cleared out the top seeds.
Muguruza will rise to a career-high of number five having made the semi-finals, and would reach number four if she takes the title Saturday.
Williams will re-enter the top 20 after her semi-final performance, and still has an outside chance of qualifying for the season finale in Singapore if she wins at Wuhan and has a strong performance in next week's China Open.