Simona Halep will take over the top ranking after reaching the China Open final on Saturday, and Rafael Nadal will play Nick Kyrgios for the men's title.
Halep beat French Open champion Jelena Ostapenko 6-2, 6-4, ensuring that when the WTA rankings are updated on Monday she will be the first women's No. 1 from Romania.
Halep could have reached No. 1 earlier but was upset in the French Open final by Ostapenko in their first career meeting.
"Of course it's the best moment in my life, and I want just to keep it," Halep said in Beijing. "And I have a few more dreams in my career. I tell you one, only one. To win a Grand Slam."
Halep only had to reach the final this week - and not lose to Elina Svitolina of Ukraine in the final - to knock Garbine Muguruza from the top spot. Muguruza retired during her opening match on Monday because of a cold.
Top-seeded Nadal defeated third-seeded Grigor Dimitrov of Bulgaria 6-3, 4-6, 6-1 in the semifinals, his tour-leading 60th match win.
Dimitrov's only victory against the Spaniard in 10 matches came last year in the China Open quarterfinals. On Saturday, his backhand cross-court winner secured the second set, on a second set point, on Nadal's serve in the 10th game.
But Nadal took immediate control of the third set, racing to 4-0.
Kyrgios didn't drop his serve in ousting second-seeded Alexander Zverev of Germany 6-3, 7-5 in the other semi. The Australian had an impressive 70 percent first-serve percentage and posted 11 aces to none for Zverev, who is normally known for his serving prowess.
Nadal and Kyrgios are tied at two matches apiece in their head-to-head, but the Australian won their latest outing in August in straight sets in the Cincinnati quarterfinals.
Nadal is one of three players, along with Roger Federer and Zverev, to win five titles this year. He won the China Open 12 years ago.
"It's very important for me to continue with the positive feelings after winning the U.S. Open," Nadal said. "To be back here and be in the final with that very tough draw I had since the beginning is a great effort."
Looking for her second title of the year after winning in Madrid, Halep will play Caroline Garcia of France in the Beijing final. Garcia beat Petra Kvitova 6-3, 7-5 in the second semifinal.
Ostapenko entered the semis without dropping a set. But Halep dominated the first set, breaking Ostapenko's serve in the first game at love. She cemented her lead with another break in the seventh game for 5-2 to serve out the set.
In the second, Halep surrendered a 3-2 lead with a service break in the sixth game, but went on to break serve again in the ninth. At 40-0 in the final game, Halep won with a clean forehand winner down the line.
Garcia, who saved a match point against Svitolina, is on a 10-match winning streak. Garcia lost in straight sets to Halep in the Montreal quarterfinals in their last meeting.
"That's the kind of match I want to play, against the top players," Garcia said of Sunday's final against Halep. "She's a very tricky player."