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ROME — Danielle Collins has been nearly unbeatable lately. Even with her announced retirement looming.

The 15th-ranked American beat former No. 1 Victoria Azarenka 6-4, 6-3 on Wednesday to reach the Italian Open semifinals and extend her record to 19-1 since early March.

Having won back-to-back titles in Miami and Charleston, South Carolina, recently, she hasn’t dropped a set on Rome’s red clay.

Collins frustrated Azarenka with her aggressive baseline play and even took out her aggression on a TV cameraman at one point, telling him after being approached on a changeover, “Hey dude, you’re going to have to back up a little bit.”

Late in the second set, Azarenka slammed her racket on the Campo Centrale as her unforced errors piled up. Then Collins slammed her racket, too, after wasting a series of break points. But Collins came up with some clutch serving to finish Azarenka off — finishing with six aces to Azarenka’s none.

Collins’ semifinal opponent will be Aryna Sabalenka, who showed no ill-effects from her previous marathon match as she eased past Jelena Ostapenko, the 2017 French Open champion, 6-2, 6-4.

All three top-ranked women have reached the final four in Rome as the other semifinal pits the top-ranked Iga Swiatek against No. 3 Coco Gauff.

In men’s action, 29th-seeded Alejandro Tabilo of Chile extended his surprising run by beating Zhang Zhizhen 6-3, 6-4 to follow up his upset victories over Novak Djokovic and Karen Khachanov in the previous two rounds.

Tabilo, who hasn’t dropped a set during his first appearance in Rome, served an ace down the T on his first match point.

In his first Masters series semifinal, Tabilo will face either 2017 champion Alexander Zverev or 11th-seeded Taylor Fritz, who were playing later.

The other quarterfinal matches are scheduled for Thursday. Stefanos Tsitsipas faces Nicolas Jarry — another Chilean — and Hubert Hurkacz plays Tommy Paul.

Sabalenka, who didn’t allow the ninth-seeded Ostapenko a break opportunity, showed no sign of the back problem that required treatment during Monday’s three-setter against Elina Svitolina that ended well after midnight local time.

The 26-year-old Sabalenka said she didn’t even practice on Tuesday and focused instead on treatment and recovery. And that clearly paid off.

“I would definitely say that it was the best performance of the tournament,” Sabalenka said. “I’m super happy that I was able to play without pain today and hopefully with every day I will feel better and better, and I will recover as good as I can for the next match.”

Rome is the last big warmup tournament on clay before the French Open starts in 11 days.