Serena Williams then beat Olga Govortsova, 7-5, 6-0, to give the United States a 2-0 lead in the best-of-five event going into today’s matches.

Serena Williams is scheduled to play Azarenka - if she’s healthy - at noon, followed by McHale vs. Govortsova and Venus Williams-Liezel Huber vs. Azarenka-Govortsova. The US team needs to win just once today to return to the World Group with a chance to compete for the Fed Cup title in 2013.

McHale said she didn’t change her preparation after being told this morning that Azarenka had withdrawn with a back injury.

Azarenka, who catapulted into the world’s top spot after winning the Australian Open, said she was feeling the effects of the last month.

USTA officials said Azarenka has spent more time getting therapy for her back than practicing while she has been here.

“I had the pain since the Australian Open and was hoping to recover, but did not have the time with all the flying I did,’’ Azarenka said in the statement.

At 4-4, McHale fought back nerves and a few close line calls to finish the match with tough serving against the 25-year-old Belarusian for her first Fed Cup victory. She had been 0-3.

McHale, 19, had Yakimova on the defensive nearly constantly with forehand slams from the baseline, sharply angled forehands that drew the Belarusian wide off the court, and deft, low approach shots. Yakimova became frustrated early, slamming loose balls across the court and bounding her racket at one point.

“I started to feel the nerves when I had the big lead in the second set,’’ McHale said.

US captain Mary Joe Fernandez steadied McHale at the net during the final two games.

“She told me she wanted me to keep my feet moving a lot and to keep swinging out like I did at the beginning,’’ McHale said. “She helped me to stay calm.’’

Serena Williams dominated most of her match. Williams, ranked No. 12 in the world and playing her first Fed Cup in five years, showcased a power serve and equally effective overhand to make quick work of Govortsova in the second set after a thrilling first-set win.

“[Govortsova] played really well in the first set, and I think I could have played better,’’ Williams said. “In the first set, I made a few mistakes, and in the second set, I think I worked to correct them.’’

Williams looked sharp early in the match, storming out to a 3-0 lead. Govortsova, though, rallied to win the next three games - but the joy of victory was fleeting.

Williams won the next game in emphatic fashion, taking the first point with a nice forehand winner, followed by a loud scream that got the crowd on its feet. Moments later, Williams won the most exciting point of the afternoon.

Govortsova hustled to her left along the baseline and ripped a crosscourt shot that Williams tracked down and lobbed over the net. Govortsova returned, Williams guessed correctly, darting to her left and hammering a winner that had the crowd again standing, this time in appreciation of both players’ effort.

“It feels really good to be playing [for the United States],’’ Williams said. “I love that the crowd can come out and cheer for their country in events like this.’’

The first set was tied, 5-5, but Williams won the next two games, and seemed to deflate Govortsova, who was bowled over in the second set.

Williams, who acknowledged she was still nursing a left ankle injury suffered in Australia, at times played inconsistently, but when she needed a point, Williams relied on her power game to overmatch the Belarusian.

“The ankle is feeling fine,’’ Williams said. “I’m really surprised at the progress it’s made, and each day it gets better.’’

The second set didn’t feature as much drama, as Williams steamrolled, showing her trademark blend of speed, power and precision shot-making.