THE FANTASTIC FOUR

KEYS, COCO FILL OUT ALL-AMERICAN SEMIS

And then there were four. Madison Keys joined the surging U.S. brigade in an all-American women’s semifinal foursome when she pummeled qualifier Kaia Kanepi 6-3, 6-3 in a swift 1 hour and 9 minutes during Wednesday night’s quarterfinals.

It’s the first time since 1981 — when Tracy Austin, Chris Evert, Martina Navratilova and Barbara Potter were the fearsome foursome — that all four Open women’s semifinalists were Americans. It also assured the first all-American Open final since 2002 when the Williams sisters faced off.

Coming off a series of winter injuries, including two surgeries on her right wrist, Keys came into the Open in good form after excelling at the August tune-ups, winning the event in Stanford. But she never expected this — her first U.S. Open semifinal.

Keys had her second wrist surgery after losing in the French Open in the second round. She was so “nervous’’ entering Wednesday’s quarterfinal she texted her coach, Lindsay Davenport, to have a good speech prepared.

“Lindsay even said to me today, she said, ‘If I told you after Paris that you’d be a quarterfinalist at the U.S. Open, would you believe me?’ I said, ‘There’s no way,’ ’’ Keys said. “All of the things that were thrown at me this whole year and having some really low moments. There was a moment where I came off the court, and I said, I don’t know if I’m ever going to win a tennis match again. Having to go in for a second [surgery] was obviously really scary.”

Her wrists look good now. She’s got ferocious groundstrokes and her two-handed backhand was particularly piercing against Kanepi. CoCo Vandeweghe now awaits the 15thseeded Keys, who banged out eight aces Wednesday. Keys has a 2-0 edge — both victories coming this summer, in Stanford and Cincinnati.

This means the world to me,’’ Keys said of making her first Open semi. “There’ll be a lot of big serves and I don’t think many long points. Sorry guys.’’

Kanepi, 32, entered the Open ranked 418th and has been limited the last two years because of plantar fasciitis.

Wednesday’s was Keys’ first Open quarterfinal appearance after bowing out in the fourth round the last two years — to Serena Williams and Caroline Wozniacki. Keys has been to one Grand Slam semifinal — the 2015 Australian Open.

“There was definitely a lot of dark moments,’’ Keys said. “To have this really feels good and makes me really proud of myself.’’