After serving her 13th ace to close a 6-3, 3-6, 6-1 victory, Williams buried her face in her towel and sobbed from her courtside chair. She was still crying as she left the court, overcome with emotion after enduring a long layoff that included two foot operations and treatment for blood clots in her lungs.
"I usually don't cry … but it's just been so hard," the four-time Wimbledon champion said. "I never dreamt I would be here right now. And then to win. I just wanted to win at least one match here."
Following Williams on Centre Court was Roger Federer, who began his chase for a record-equaling seventh Wimbledon championship by beating Mikhail Kukushkin of Kazakhstan 7-6 (2), 6-4, 6-2 in the wind.
Other winners included three-time Wimbledon runner-up Andy Roddick and Juan Martin del Potro. Among the women, top-seeded Caroline Wozniacki made a strong start in her latest bid for her Grand Slam title while former No. 1 Jelena Jankovic lost in the first round,
Williams fought through a midmatch slump, then reasserted command over her French opponent in the third set with a big serve and powerful groundstrokes. She showed why she is still considered a title favorite despite her long absence and her No. 7 seeding.
"It's been a disaster year, but I've been praying," Williams said. "To be able to come back at Wimbledon is pretty awesome. I didn't expect to play. And I didn't expect to even do anything. So I'm just excited. I've never cried with joy for anything."
After losing the second set, Williams took charge in the final set, winning the last five games.
"I kept thinking, 'This is Wimbledon,'" she said.
The point of the match came at 3-1, when Williams stretched for a backhand at the baseline and fell over as she hit a winning lob over Rezai at the net. Rezai said she saw the tears in Williams' eyes when they shook hands after the match.
"It definitely was so emotional for me because throughout the last 12 months, I've been through a lot of things that's not normal, things you guys don't even know about," Williams said. "It's just been a long, arduous road. To stand up still is pretty awesome."
In keeping with Wimbledon tradition, Williams opened play on Day 2 on Centre Court as the women's defending champion. She strode onto court wearing a cardigan, and played in a classic dress with blue trim. Her fingernails were painted in the Wimbledon colors of purple and green.
The 61st-ranked Rezai came out hitting hard, breaking Williams in the first game that lasted nine minutes and going up 2-0. But Williams then won five games in a row and took the set with only four unforced errors. Williams lost the momentum in the second set, and Rezai broke in the sixth game to force a third set.
Williams said it was the most emotional she's ever felt after a victory — and this was only a first-round match.
"For me it wasn't about winning the match," she said. "It was about being out there. … It just really goes to show if you don't give up, you still have a chance. I guess I proved that I could, that I could. I think that sums it up: I could."
Federer never lost serve, saving all three break points, and won 29 of 31 of his points on serve in the first set. The third-seeded Swiss piled up 53 winners — 37 more than Kukushkin, who was making his Wimbledon debut.
Federer, a 16-time Grand Slam winner who lost in the Wimbledon quarterfinals last year, is looking to equal Pete Sampras' seven titles at the All England Club.
"The first match at Wimbledon is never easy really because we don't get any practice time on Centre Court," Federer said. "I thought I struggled putting enough returns into play in the first set but I thought he played well. For a set and a half he made it hard for me. I also served really well so I was very pleased with the match."
Roddick served 30 aces — including four in a row in one game — and beat German qualifier Andreas Beck 6-4, 7-6 (6), 6-3 to reach the second round.
Del Potro, the 2009 U.S. Open champion, downed Flavio Cipolla of Italy 6-1, 6-4, 6-3. The 24th-seeded Argentine led Cipolla 6-1, 1-3 when rain suspended play Monday, then won 11 of the 15 games played Tuesday.
On Court 1, Wozniacki defeated Arantxa Parra Santonja of Spain 6-2, 6-1. The 20-year-old Dane had fewer winners than her opponent, but made only five unforced errors.
Jankovic was beaten by Maria Jose Martinez Sanchez of Spain 5-7, 6-4, 6-3. The 15th-seeded Serb reached the fourth round at the All England Club last year.
Also advancing among the women were No. 4 Victoria Azarenka, No. 8 Petra Kvitova, No. 11 Andrea Petkovic and No. 14 Anastasia Pavlyuchenkova. Men's winners included No. 15 Gilles Simon and No. 21 Fernando Verdasco.
Serena and older sister Venus have won nine of the last 11 Wimbledon titles. Venus, a five-time champion who was off for nearly five months with a hip injury, is in the opposite half of the draw and could face her sister again in the final.
Venus next faces 40-year-old Kimiko Date-Krumm, who is back in the second round at Wimbledon 15 years after she reached the semifinals and three years after coming out of a long retirement.
"It was amazing when she came back," Venus said. "I watched her results. She's been playing so well. I always root for her actually. But this time I'm playing her, so I will be rooting for me this time."