Serena Williams overcame foot surgery and blood clots on her lungs in the past two years to regain the form that has made her a 14-time Grand Slam tennis champion.
Now her rivals and admirers from men's tennis see her as the unmatched queen of the sport and a clear favourite entering Monday's start of the US Open.
"It's incredible. Nobody knows where she is going to stop and she keeps on going and dominating," said defending US Open men's champion Novak Djokovic.
"I'm happy I'm in men's tennis -- not needing to face her."
Williams won last month's Wimbledon title, her first Grand Slam crown since her health issues arose after a 2010 Wimbledon title, and followed up by taking the London Olympic gold medal on the same All-England Club grass courts.
"She has had some issues. That makes the return to the top of the rankings and the top of her game even more exciting," said World No. 1 Roger Federer, a 17-time Grand Slam champion.
"It has been great what she has done over the last 15-plus years but it's an amazing summer for her. She played great. If she's on she's very hard to beat. I think she just proved that point again."
"What we have seen over the last few months is the best player ever," said Kim Clijsters, a four-time Grand Slam champion set to retire after the US Open.
But it was a rise from the lowest of depths.
Williams needed 18 stitches and surgery after stepping on broken glass in a Munich restaurant in a celebration of her 2010 Wimbledon triumph.
Then she suffered life-threatening blood clots on her lungs and thought her time at the top was over, making her comeback to win at Wimbledon even more astounding and her favoured status at Flushing Meadows a certainty.
"She gained a tremendous amount of confidence at Wimbledon," four-time Grand Slam champion Maria Sharapova said. "When she got to the Olympics with every match she just improved.
"She took that confidence and played just really great physical tennis, served extremely well. Obviously she's the favourite because she won those two big events back to back."
Williams, whose sister and doubles partner Venus is a seven-time Grand Slam champion, will turn 31 next month. That's the same age her elder half-sister Yetunde was when she was killed in a shooting in 2003.
"Every day people inspire me," Clijsters said. "Serena is one of those with what she has been though, not just with the health problems but dealing with the loss of their sister and all those kinds of things."
Williams saves herself to play her best at the events that matter most.
"Physically now at this stage of her career she cannot maybe travel full time on the tour but she's always performing her best in majors," Djokovic said. "She's fantastic."
Even the Williams sisters must call it quits at some stage, although Venus said she hopes Serena will retire at the same time she does and Serena hopes Venus, battling an auto-immune disease, will hang around a while longer.
"I would have to go on if she decided to retire tomorrow," Serena Williams said. "I would be devastated. I can't get a better doubles partner.
"If it was that soon I would have to go on, but if it was a little later, maybe it will be together."