MELBOURNE, Australia — Though Serena Williams does not have the physique of a contender, she still has a champion’s heart. Nadia Petrova, we learned once again yesterday, does not.

The powerful Petrova, seeded no. 5, had Williams dead to rights in the third round of the Australian Open on a rainy Friday morning, when she served for the match at 5-4 in the second set, the roof closed above her head. But she lost six consecutive games as Williams cut down on her errors (18 in the first set, 21 in the next two) and Petrova tensed up (four errors in the first, 24 the rest of the way). The American closed the match with an ace, an overhead, and a service winner, 1-6, 7-5, 6-3.

As Williams fights to stay alive in Melbourne, James Blake is hunting bigger game.

He has made remarkable progress these last few years, overcoming a broken neck, a bout with shingles that paralyzed half his face, and the death of his father to earn a place among the top five tennis players in the world for the first time in his career. Since his comeback began in January 2005, Blake, 27, has won eight titles — seven more than he won in his previous six years as a professional.

But there’s one thing Blake has yet to do: reach a semifinal or final of a Grand Slam. At this year’s Australian Open, he has the best chance of his career to do it.

The first weekend of the first major of 2007 is here, and Blake says he’s not looking ahead in the draw. If he did, he would like what’s in front of him. Next up in round three is his friend Robby Ginepri, a steady baseliner who jumped almost 90 places in the rankings, to no. 15, in 2005 before crashing back to the 50s last year. Blake knows Ginepri’s game well, and he beat him during their last meeting, on grass, in 2006.

The fourth round will present a bigger challenge, at least on paper: the winner of local favorite Lleyton Hewitt and Chilean Fernando Gonzalez, whose new coach, Larry Stefanki, has helped transform “Fernando Forehand” into a versatile, and less reckless, player. Blake has had troubles with both men, especially Gonzalez, who has won their last three meetings (the most recent on grass in the Davis Cup). Hewitt and Blake have a long history, and Blake hasn’t fared well. The Australian won their first six meetings before Blake defeated him in a hard court final in Las Vegas last year. Hewitt also won on grass in June.

At the moment, though, Blake looks far better than both of these men. He has won his first six matches of the year, including a title in Sydney, and hasn’t dropped a set at this tournament. Gonzalez struggled mightily on Thursday against Juan Martin Del Potro, a rangy, 6-foot-5-inch Argentine with a bright future playing his fourth career match in a Grand Slam event (Gonzalez led 7–6(7), 4–6, 6–7(3), 6–4, 4–0 when Del Potro meekly retired because of cramps). Hewitt needed five sets to defeat American Michael Russell in the first round and also dropped a set to mediocre Canadian Frank Dancevic, who had advanced to the second round of a major for the first time in his career.

When Blake brings his nimble feet, leaping overheads, and booming forehands to the court this weekend, he’ll be the favorite no matter the opponent. Ginepri, who defeated left-handed serve and volleyer Mischa Zverev in straight sets on Thursday, explained why.

“He’s got it going for him right now,” Ginepri said. “He’s hitting his backhand a lot better. Before you could hit a few balls to the backhand and he would give you a short one, but now he can pull it up the line a lot better, and he can stay in the backhand rally crosscourt a little bit longer than he could before. His serve is a little bit bigger too.”

So far, Blake has served consistently: 59% first serve percentage in his two matches, with a winning percentage of 79% (both numbers are a tad better than his stats for the 2006 season). On his second serve, Blake’s performance over two matches is far above his level in 2006. He won 52% of his second-serve points last year; so far this tournament, he’s winning 69%. If Blake plays under a closed roof, as he did in his first two matches, those numbers will be easier to maintain.

In tennis years, 27 is prime, perhaps even past it, though Blake’s body seems to have benefited from the time off caused by his many misfortunes. He hopes he can seize the moment.

“I know anything can happen,” he said. “I can keep rising in the rankings. I could dip real low. I could get injured. I could just kind of stagnate. I could stay in the top 10. I have no idea what’s going to happen.”

If Blake continues to thrive, he might face Rafael Nadal in the quarterfinals. Blake has had the Spaniard’s number, defeating him in all three of their meetings. Late Thursday evening, Nadal absorbed some punishment from Philipp Kohlschreiber, a freeswinging German who twice hit Nadal with a ball when the 20-year-old Spaniard was at the net (the second deflected off Nadal’s racket as he went down; he gave Kohlschreiber an annoyed glance). Over the three-and-ahalf-hour match, Nadal did 20% more running than his opponent, according to data collected by Australian television. But Nadal likes to work, and he sounded confident after his 7–5, 6–3, 4–6, 6–2 victory.

On the other half of the men’s draw, world no. 1 Roger Federer methodically works his way toward his eleventh major final (he’s won nine of them). The rest of the women’s draw is taking shape, too, with 24-year-old Kim Clijsters, in her last year as a pro, playing the most impressive tennis in two decisive victories (she has lost three games total). One other American remains: Ashley Harkleroad, a native of Georgia whose brief marriage to fellow tennis player Alex Bogomolov ended in divorce, reached the third round on Thursday.

tperrotta@nysun.com