Cincinnati - Angelique Kerber moved to the brink of the No 1 ranking, holding off a charge from Simona Halep with a 6-3, 6-4 win to reach the final of the WTA Cincinnati Masters.
The German Olympic silver medallist is poised to replace Serena Williams on the top spot if she can defeat 15th seeded Karolina Pliskova after the Czech upset fourth seed Garbine Muguruza of Spain 6-1, 6-3.
"There is still one match to go and I will not think yet about this," the 28-year-old Australian Open winner said after reaching her second final on the US midwest hard courts.
"If it happens, it will be amazing. I have to focus on a match against a tough opponent."
Williams has been the No 1 ranked player for 183 straight weeks.
Kerber played for the 2012 Cincinnati title, losing to China's Li Na. She stands 4-2 over Pliskova, whose big serve produced her 400th ace of the season in the final game of her upset victory over Muguruza.
Kerber and third seed Halep - winner of her last 13 matches - were delayed an hour by rain and then interrupted for 30 minutes.
The back-and-forth match was littered with 16 breaks of serve from a combined 24 chances.
Kerber won the opening set and took a 4-0 lead in the second as Halep faded in the long rallies after a slow start.
But after missing a point for 5-1, Kerber had to work as her opponent came alive, pulling to 4-5 as she got one of two breaks back.
The German held on in the next game, setting up a match point and taking the win despite breaking a string on the final point.
"I was just hoping the ball would fly in," Kerber said. "I'm so happy to have beaten such a tough player. I tried to stay positive and think that I could win.
"I was feeling my rhythm and believed in myself. It feels good to be in the final again."
At 17 in the world, Pliskova is the lowest-ranked finalist at Cincinnati since 2008, when No 21 Natalie Petrova won the title. Muguruza was overwhelmed with just seven winners and 23 unforced errors.
"I'm happy with how I played today," said Pliskova. "The whole week it has been hot and humid. Today it was windy and not that hot.
"I'm glad to win a good match. I tried to play aggressive, she has a similar game to me.
"I knew I had to make the pressure and concentrate on my serve."