Dana Vollmer wins the 100-meter butterfly final Thursday at the Arena Pro Swim Series in Mesa, Ariz. (Jonathan Newton/The Washington Post)

The comeback started with a huge splash, a gasping for air and a realization: Dana Vollmer had a long way to go. In late-April 2015, two years after taking a hiatus from swimming and seven weeks after giving birth to her first child, the four-time Olympic gold-medal winner dived back in the pool for her first workout and could barely make it through a 300-meter warmup. She felt, she would say later, like a “whale with toothpicks for arms.” The 2016 Olympics felt like a galaxy away.

Twelve months later, Vollmer may be the best story in American swimming: a 28-year-old mom who stepped away from the sport after the 2013 world championships in Barcelona, only to change her mind while on bed rest eight months into her pregnancy, and who is now posting some of the best times of her life — and in the world. Rio de Janeiro no longer feels so far away. It is, in fact, within an arm’s length.

“It’s not that I have to make the Olympic team to prove anything,” said Vollmer. “It’s that I so want to be there. I love walking out for finals and looking at the pool. I had one of those moments [here]. It was so pretty staring at that still water. In my career, I’ve struggled with pressure and expectations — so to be able to come full-circle and love every time I get to be in the pool has been a big blessing.”

Vollmer’s first two events this week at the Arena Pro Swim Series stop in Mesa — the biggest American meet leading into June’s Olympic Trials in Omaha — have both felt like game-changers for American swimming.

Dana Vollmer shows off her gold medal in the 100-meter butterfly at the 2012 London Olympics. (Christophe Simon/AFP/Getty Images)

On Thursday night, in the finals of the 100-meter butterfly, Vollmer ran down the reigning American and NCAA champion, Kelsi Worrell, in a time — 56.94 seconds — that ranked as the fastest by a U.S. woman since Vollmer herself won in London with a then-world-record time of 55.98. Then, in Friday’s 50-meter freestyle, she swam a career-best 25-flat in her preliminary heat to qualify second overall, and a wind-aided 24.69 — the 10th-fastest in the world this year — in the evening final, finishing second to Madison Kennedy (24.45). Taken together, the two strong freestyle swims further validated her plans to swim the event at Omaha.

“I’m fighting the clock. Every week counts, heading into Rio,” Vollmer said. “A year ago, I was wondering, ‘Could I get back in good enough shape to get a spot on the team?’ And now I’ve gotten there. I’ve gotten faster than I thought I would.”

As she spoke to a handful of reporters Thursday evening, just after her win in the 100 fly, Vollmer balanced 13-month-old Arlen first on one hip, then the other. When Michael Phelps, himself a soon-to-be father, came into the same room moments earlier, he greeted Vollmer with a giant smile. “Nice swim, mama,” he said.

Vollmer said she had planned on leaving Arlen home with her husband in Danville, Calif., and come to Mesa by herself. “I wanted to just come and be focused [on] the meet,” she said. “But I had a moment of — ‘No, I need to bring you.’ It was the day before I left. ‘I can’t do it. I have to bring him with me.’ I love it. It keeps things in perspective for me. He was just sitting there smiling. He doesn’t care if I have a bad race.”

In a roundabout way, Arlen is the one who pushed Vollmer to this place. With a month or so to go in her pregnancy, having put on 50 pounds, she was ordered to bed-rest. For a lifelong elite athlete — one who had qualified for the 2000 Olympic Trials as a 12-year-old and won a relay gold at the 2004 Athens Games at 16 — it felt like a prison sentence.

“I was getting antsy,” she recalled. “All I wanted to do was move.” Then and there, she resolved to come back.

The road back wasn’t as long as it might have been: When she decided to take her break following the 2013 world championships — and unlike Michael Phelps, who stepped away the year before — she notably declined to sign the official papers that would have taken her out of the sport’s drug-testing program and made it more difficult to return.

“To actually not swim — I really started to realize what I missed,” she said. “Was it really that bad, working out for a living all the time? It helped me in coming back and appreciating all the things I had started to resent, like being tired all the time and waking up early in the morning.

“That’s how it started: ‘If I want to get my body back, what better way to do it than to see what I can do?’ ” she said. “Then it became, ‘If I get back in shape, how fast could I go in the 100 fly?’ No one in America had gone as fast as I’d gone [in London] yet.”

She gave birth on March 6 and was back in her home pool at the University of California-Berkeley seven weeks later, with her old coach, Teri McKeever, on the deck. She described those initial workouts as “humbling,” but by August — less than four months after starting up again — she was swimming the Phillips 66 U.S. Nationals in San Antonio and finishing fourth in the 100 butterfly.

“We never know if we’re going to get a good night’s sleep or not,” she said of the challenges of competing while traveling with an infant. “He’s just throwing in some more variables for me to learn how to deal with. . . . I feel like there’s not much you can throw at me that I won’t be able to handle”

Worrell, the butterfly phenom who once got Vollmer’s autograph at a meet she attended as a kid, is among those both amazed and emboldened by the latter’s return to the top of the sport.

“It’s incredible,” said Worrell, a senior at Louisville. “What she’s doing hasn’t been done very often. It opens a lot of doors for other women to realize, ‘Hey, if she can do it all — be a mother, a wife and a professional swimmer — what can I do, too?’”

Vollmer has had a few long talks with Worrell over the past few months, answering questions and offering advice on the process of turning pro and juggling the different pressures of adulthood. But on Thursday night, when she lined up in the lane next to Worrell in the 100 fly finals, she had only one thought: “I wanted to beat her.”

And then she did exactly that, her time suggesting the old Dana Vollmer was all the way back, but her immediate goals — drop another second between now and August and challenge for the gold in Rio — suggesting the new Dana Vollmer can be even better.