DOHA, Qatar – Serena Williams appeared unstoppable in the summer of 2010; the American had just won her fourth Wimbledon title and was the undisputed World No.1 when an out-of-nowhere foot injury ended her season.

“I left ranked No.1,” Williams said nearly a year later in Eastbourne. “That’s what I miss most, just being on top of the game and just playing some really good tennis, the challenges of all the players.”

Things went from bad to worse when her return to the game was further delayed by breathing troubles that turned out to be something even more serious: a pulmonary embolism.

“I honestly just thought I was out of shape, that I needed to get on the treadmill or something. They just said it could have gotten a lot serious a day later or two days later. It could have been really not good.

“It could have possibly been career-ending, but for the grace of God I got there in time and I was able to recover from it.

“I’m just taking it one day at a time. I mean, I’m not just preparing for today or Wimbledon. I’m preparing for the rest of my career.”

From a nadir of No.172 in July of 2011, Williams went on a tear that summer, winning 18 straight matches to reach the US Open final.

Clicking into gear with gusto in 2012, the American reclaimed her Wimbledon crown – her first major title in exactly two years – added an Olympic Gold medal at the Summer Games in Lodon, and capped a near-perfect season with wins at the US Open and WTA Finals.

She came into that next year’s Qatar Total Open having won 56 of her last 59 matches, with a run to the semifinals all she needed to return to No.1. From 4-1 down in the final set, Williams roared past Petra Kvitova 3-6, 6-3, 7-5 in the quarterfinals to cap an emotional comeback.

“I don’t know how I did it – I really don’t know,” she said after the match. “I just hung in there and she was playing so well. Every time I looked around she was hitting a winner.

“I just tried to stay in there.”

Williams has been atop the WTA rankings ever since, adding six more majors to her current total of 21, and is set to pass Martina Navratilova for consecutive weeks at No.1 at 157 straight weeks.

Steffi Graf remains the final frontier for the American, is in position to pass the German’s haul of 22 major titles and 186 straight weeks at No.1.

“In my particular situation, I never thought I’d play again,” she told press that night in Doha. “Then I thought I’d never be able to win tournaments or Grand Slams. No.1 was so far off. It was always a dream, but, you know, I was No.1 when tragedy struck, and it was just an awful thing to happen.

“So I’m happy that I’m back.”

All photos courtesy of Getty Images.