Top seed and defending champion Sabalenka had no answer for Elena Rybakina, falling to the 2022 Wimbledon champion 6-1, 6-4.

Reigning French Open champion Gauff had 16 double faults in a 2-6, 6-4, 6-3 exit at the hands of seventh-seeded Italian Jasmine Paolini.

Spain's second-ranked Alcaraz had his difficulties, but he broke Rublev in the final game of a tense duel to emerge a 6-3, 4-6, 7-5 winner and will face third-seeded Alexander Zverev, who brought fifth-seeded Toronto champion Ben Shelton's run to an end with a 6-2, 6-2 victory.

Zverev earned a 4-1 lead in the second set as Shelton angrily tossed his racquet on the changeover, and the 2021 champion polished off the win on a second match point. He now has 24 hours to try to get fit.

"I'm not feeling too great now, but I have a day to get fresh," Zverev said. "I hope to be at 100%.

"I'm not sure what happened. I felt very well, but at the end of the first set, it got progressively worse. But I'll do all I can to give it a shot tomorrow."

Alcaraz was broken while serving for victory at 5-4 in the final set against 11th-ranked Rublev, but he found his focus and used his trademark grit to come through three games later.

He sealed the triumph as an emotional Rublev hit an eighth double-fault on match point.

"I maintained the positive thoughts even if I lost focus a bit in the second set," said Alcaraz, whose five titles so far this year include the French Open as well as Masters 1000 trophies in Monte Carlo and Rome.

- Rybakina revenge -

Rybakina fired 11 aces en route to a fifth career victory over Sabalenka in 12 meetings.

She avenged a loss to the Belarusian in Berlin two months ago and booked a semi-final meeting with Wimbledon champion Iga Swiatek -- a 6-3, 6-4 winner over Anna Kalinskaya.

Rybakina was aided by 11 aces as she reached her first Cincinnati semi-final.

"I'm happy with the serve. It was the key today," Rybakina said. "I served really well."

"If she had been serving well, it would have been a totally different match," she acknowledged.

Rybakina has won her last three matches against Swiatek, all played this season.

But Swiatek was feeling confident after downing Kalinskaya in what she called "the best match I played here."

"I'm happy that I'm progressing during the tournament," Swiatek said.

Gauff, the 2023 Cincinnati champion, had looked well on her way after a quick first set against Paolini.

But her mistakes caught up with her as she was broken eight times by the Italian, who overcame plenty of errors of her own as well as a twisted ankle suffered early in the third set.

"It was so tough at the start, I was running and trying to put the ball inside the court," Paolini said. "I started to feel the ball better."

"I fought for every point and stayed in the match," added the Italian, who will face Veronika Kudermetova for a place in the final.

Kudermetova crushed Varvara Gracheva 6-1, 6-2.