Sloane Stephens, an endearing 19-year-old with a megawatt smile, had just stunned the world — and herself — Wednesday with a 3-6, 7-5, 6-4 upset of her heroine, Serena Williams, in the Australian Open quarterfinals. Her mind turned to her bedroom back home in Coral Springs, where a poster of Williams hangs on the wall.

“I think I’ll put up a poster of myself now,” she joked on the court.

She then drew laughs explaining her attempt to reach her mother, Sybil Smith, a former college All-American swimmer, who was watching on TV from Coral Springs with her younger brother, Shawn. “I couldn’t reach my mom. So I had to call my brother, and he couldn’t even talk. He was, like, freaking out. I was, like, ‘OK, where’s mom?’ I was, like, ‘OK, never mind. Bye’… I’m sure my mom had like four heart attacks.”

Stephens, ranked No. 29 in the world, reached into her bag, pulled out a brightly colored cellphone and found hundreds of text messages. She laughed and said she might have to use a chunk of her winnings ($500,000 after Thursday’s semifinal loss to Victoria Azarenka) to pay her phone bill. “I thought it was free to receive text messages, but someone told me otherwise,” she said. “So I’m trying to figure out what to do, because my phone bill will be crazy, and my mom is going to be, like, ‘Oh my god, your phone bill.’ She’s going to be, like, ‘the money you were going to buy yourself something nice with, you’re going to have to pay your phone bill.’ ”

Stephens’ momentous win, and her down-to-earth personality, expanded her fan base. Her Twitter following (@sloanetweets) went from 17,000 Tuesday afternoon to 46,700 by Wednesday afternoon. She told reporters she’d love a call from President Barack Obama: “I really want to go to the president’s Easter egg hunt, so if you could put in a good word for me.”

On Monday, before her fourth-round match, Stephens said: “Little kids go home first week; big girls stay for the second week.’’

Rather slight by tennis standards at 5-7, Stephens is now officially a big girl. There were 17 teenagers in the draw at the start of the Australian Open. She’s the only one left, and is expected to crack the top 20 next week. Her previous best result at a Grand Slam tournament was the fourth round at the 2012 French Open.

She had lost to Williams, the 15-time Grand Slam champion, a few weeks ago in a tournament in Brisbane, Australia. Williams was on a 20-match win streak and had won 46 of her 48 matches since the Sony Open in Key Biscayne last March, including the Wimbledon title, Olympic gold medal, and U.S. Open title. Nevertheless, Stephens was not intimidated when she woke up Wednesday.

“When I got up, I was like, ‘Look dude, you can do this,’ ’’ she said, laughing. “You have to go out and play your game, no matter what. Without the titles, with the titles, it’s still a tennis match, the court’s the same size, you’re still playing a regular person across the net.”

Watching her proudly on TV from Plantation was Nick Saviano, who coached Stephens from age 11 through her pro debut six years later. Although she trains mainly at the U.S. Tennis Association center in Carson, Calif., these days, she and her mother remain close with Saviano, and she still seeks his advice. She texted him early Wednesday morning.