What sets Serena Williams apart?

NEW YORK -- Only Serena.

Who else could be off for a year, start dominating the WTA Tour within a few weeks and come into the U.S. Open as the heavy favorite?

Sure, Kim Clijsters won the U.S. Open in only her third tournament back from retirement, but that was a surprise to everyone -- including Clijsters herself. And unlike Clijsters, who had trained hard for months and came back fully fit, Williams returned in June having practiced for only a few weeks after her latest health scare. Still, the No. 28 seed is widely expected to pull off a victory at Flushing Meadows just five tournaments into her comeback.

After a fourth-round loss at Wimbledon, Williams has returned to the circuit with a vengeance, scooping up titles at the Bank of the West Classic at Stanford and the Rogers Cup in Toronto before pulling out with a sore toe at the Western & Southern Open in Cincinnati. She came into the U.S. Open on a 12-match winning streak not counting the walkover in Cincinnati, and swept through a potentially tricky first-round match against up-and-comer Bojana Jovanovski 6-1, 6-1.

"She's a different player from anyone I've played before," said one of her early victims this summer, Sabine Lisicki. "I've never played such a strong player like her before. I'm hitting the ball pretty hard, and I'm not used to that ball coming back that hard. That was something new for me."

What sets her apart? Well, only Serena has a serve like Serena's. She dominates the tour's serving statistics with most service games won (87 percent), first- and second-serve points won (76 percent and 54 percent, respectively) and break points saved (70 percent).

"She has very good serve," Maria Kirilenko said. "At Wimbledon, when I played her, I couldn't break it also, so that was a big advantage for her. But her returns are good as well."

It adds up to being the best at taking the first strike, which is where she truly stands out. "From the baseline I don't feel too much pressure, so I can play the rallies," said Kirilenko, who has faced Williams twice since she came back. "The important [thing] is to receive serve, to receive the return of serve."

And once Williams takes control with a big serve or groundstroke, her power off the ground rarely allows opponents back in.

But it's not just technique and muscle -- Williams also has a mental edge. She does not doubt herself, and those on the other side of the net know how fiercely she can compete.

Only Serena can intimidate a multiple-Grand Slam champion like Maria Sharapova or a fearless upstart like Victoria Azarenka simply by her presence on court.

"Well, of course she's a great champion," Kirilenko said. "She won so many tournament[s], she has [a lot of] experience, she didn't play for one year [and] she's coming back and winning tournaments -- you have to be tough to do this stuff. It's important not to think about it."

Easier said than done.

Still, things don't come easily on a women's tour that is more competitive than ever -- even for Serena.

Williams has begun putting in some serious training, not something she has been known for in the past. For the first time, she says, her regimen includes daily practices -- something few other players would dream of not having during competition periods. She is also continuing to work with well-known trainer Mackie Shilstone, who has trained several top athletes and said that Williams would "put football players to shame."

Next up for Williams is Michaella Krajicek, the half-sister of former Wimbledon champ Richard Krajicek who has been having a much longer, bumpier comeback from injury and is not expected to give the three-time U.S. Open champion much trouble. Then would come a potential third-round match against Azarenka and possible matchups against resurgent Serbs Ana Ivanovic and Jelena Jankovic, then Francesca Schiavone.

But the draw is also opening up for Williams. Clijsters, the only other heavyweight in the game these days, is missing with an abdominal tear. Li Na and Petra Kvitova, this year's other Grand Slam winners, are out. Note that their games bear some similarities to those of countrywomen Jie Zheng and Lucie Safarova, respectively. Zheng has given Williams issues this summer by hitting the ball very early and flat, not leaving Williams enough time to unleash her strokes, and Safarova has troubled her with her aggressive lefty game. Finally, dangerous players such as Sharapova, Marion Bartoli, Sam Stosur and Nadia Petrova are on the other side of the draw. Injury may be Williams' biggest threat until the final, given that she is prone to problems and has played more matches than usual this summer.

Williams' previous appearance at the U.S. Open ended with an infamous outburst at a lineswoman over a foot-fault call, earning her a point penalty on match point. But she returned to a rousing cheer Tuesday night, and her recent woes may have superseded that incident and made her a more sympathetic figure to fans.

In any case, she has made light of the controversial episode the past few days. After her opening win, she said, "I feel fine. Even last time I played here, I went out with a bang. I came in with a bang tonight."

Given the rash of upsets and close matchups so far, can anyone really be expected to mow down the field and bring a little order to the women's game?
Only Serena.

Kamakshi Tandon is a freelance tennis writer for ESPN.com.