Wimbledon - Emotional Serena wins on return
The 29-year-old seventh seed, whose participation in the tournament was in doubt after 49 weeks out with serious health problems, flirted with a shock exit before her trusty serve and baseline power came to the rescue as she completed victory in one hour 36 minutes.
After ending the Centre Court contest with her 13th ace the four-times champion buried her head in her towel and wept and she was still teary-eyed in a televised interview at courtside. "It's been so hard, I never dreamt I would be here right now," said the American, who is bidding to become the first woman since Steffi Graf in 1993 to win three consecutive Wimbledon singles title.
"I just wanted to win just one match here seeing as I'm not playing doubles, it was just a really big win for. It's been so hard and a disaster year for me, but I've been praying and I have my family here and I just love tennis," added Williams who suffered life-threatening blood clots on her lungs in February.
It was only her third competitive match since last year's final victory over Vera Zvonareva, two weeks after which she gashed her foot in a Munich restaurant and needed 18 stitches and then surgery to repair a partially severed tendon.
As welcome backs go it was a tough one against an opponent who was once ranked 15th in the world but has slipped back after her own off-court traumas.
After warming up in a white cardigan that would not have looked out of place on the England cricketers in the Royal Box, Williams endured a torrid opening service game.
Apart from the odd rain drop, trouble with her silver hair clip, an errant forehand and some foot faults she also could have done with earplugs to blot out the awful din from an alarm that blared out across Centre Court.
After a nine-minute battle Rezai broke serve with a deft drop shot and the Frenchwoman then held to lead 2-0 to set the 15,000 crowd abuzz with the inkling of a major shock.
Williams later said the only thing that could faze her was "facing Rafael Nadal at Roland Garros" and as the looming clouds moved away and the sun broke through former world number quickly regained the old swagger.
Moving smoothly in what she called a 60s-style "baby-doll" she reeled off the next five games with the effortless power that has become her hallmark.
Rezai, who came close to beating Williams in their only previous encounter, hit back to break in the sixth game of the second set courtesy of a Williams double fault.
Williams has never suffered a first-round loss in 43 previous grand slams and she showed why in the decider with some clinical tennis to stop the charging Rezai in her tracks.
By the time the 13-times Grand Slam champion spoke to the media later she had regained her usual poise.
"I'm hoping I'll play better," she said. "Now I feel like I can take a deep breath. I've been practising better than I played today. Hopefully I'll get back to the right things."
With a degree of mystery still surrounding the exact circumstances of her foot injury, reporters' questions also veered off to the events that turned her life upside down.
Asked if she was going to sue the restaurant where she claims she trod on broken glass, she was in charitable mood.
"I've never been like a greedy person," she said, although her appetite for winning titles remains insatiable.
Williams will next face Simona Halep, after the Romanian beat promising Serb Bojana Jovanovski 6-1 6-2.