Defending champion Serena Williams recovered from a poor start to defeat Simona Halep 3-6, 6-2, 6-1 Thursday and move into the third round of Wimbledon, staying on course for a fifth title.

After dropping the first set, Williams regained her renowned intensity and powerful shot-making to dominate the rest of the way on Court 2. The winner of 13 Grand Slam singles titles is still searching for her form after a yearlong absence because of injuries and health issues.

There were no tears this time from Williams, who sobbed with relief Monday after winning her opening match on Centre Court against Aravane Rezai.

"I'm just happy to be playing and hopefully I'll get better as the tournament goes on," Williams said. "It was a little windy out there and I just was a little tight, so I just got to relax and enjoy myself more."

Williams wasn't happy about playing out on Court 2, rather than Centre Court or Court 1. Her sister, five-time champion Venus, played her first-round match on Court 2. Their two other matches were on Centre Court.

"They like to put us on Court 2, me and Venus, for whatever reason," Serena said. "I haven't figured it out yet. Maybe one day we'll figure it out. I don't know."

Williams said top male players, such as Roger Federer, Rafael Nadal and Novak Djokovic, are "never moved across" to the outside court.

The Williams sisters have combined to win nine of the last 11 Wimbledon singles titles.

"Actually Venus and I have won more Wimbledons together than a lot of the players or by ourselves in doubles even," she said. "At the end of the day, I don't know.

"They're not going to change, doesn't look like," Williams said, referring to All England Club organizers.

Told of Williams' comments, tournament spokesman Johnny Perkins said there was no intentional snub.

"I don't think it's anything deliberate, clearly," he told The Associated Press. "It's a hugely complex jigsaw puzzle. Everyone probably looks at it from their own point of view, so she's obviously quite entitled to (her opinion). ... We obviously have a duty to the paying public, plus the international audiences around the world."

From 4-2 in the second set, the seventh-seeded American won eight out of the last nine games to re-establish her supremacy on the grass at the All England Club, pumping a clenched fist and shouting "Come on!" after smacking key winners.

It was the fourth consecutive three-setter Williams has played since returning last week at the Eastbourne grass-court tournament. She had been out for nearly a year after two foot operations and blood clots in her lungs.

"I guess I just want to play longer matches because I can get more practice," she said.

In men's play, second-seeded Novak Djokovic swept into the third round by defeating South Africa's Kevin Anderson 6-3, 6-4, 6-2 on Court 1. The Serb has won his opening two rounds in straight sets after his 43-match winning streak was ended by Roger Federer in the French Open semifinals.

"On the important moments, I served well, I played well and I returned really, really well today," said Djokovic, who has twice reached the semifinals at Wimbledon and will take over the No. 1 ranking if he makes the final this year. "In the first week, you have to stay focused. You're playing lower-ranked players with nothing to lose. It's just great to win two straight-set matches."

A day after throwing his shoes in frustration, 2009 U.S. Open champion Juan Martin del Potro won 18 of 23 games Thursday to reach the third round with a 6-7 (7), 6-1, 6-0, 6-4 win over Olivier Rochus. At 6-foot-6 (1.98 meters), Del Potro stood about a foot taller than his Belgian opponent.

After the match was suspended by darkness after the first set Wednesday, an angry Del Potro took his shoes off and tossed one out of the stadium and the other deep into the crowd. When the 24th-seeded Argentine came back out onto Court 2 on Thursday, he dominated, winning 12 of the first 13 games.

As usual, Williams used her big serve to control the match. She had seven aces, won 88 percent of points on her first serve and smacked a 114 mph service winner to end the match.

Williams finished with 34 winners and 19 unforced errors, nine in the first set.

The 19-year-old Halep, making her Wimbledon debut, came out firing from the start and wasn't awed or intimidated by the occasion or her famous opponent.

With Williams spraying unforced errors, Halep played solidly and moved out to leads of 4-1 and 5-2. The Romanian tumbled awkwardly on the baseline in the seventh game, and received treatment on her left leg during the changeover. She came out and held serve, with Williams yanking a backhand wide on set point.

Williams lifted her game in the second set and won the first three games to take command. She displayed her emotions in the fifth game when, after wrong-footing Halep with a forehand winner, shrieked "Come on!" and crouched down with her fist clenched.

On set point, after a long hard-hitting baseline rally, Williams smacked a running forehand that clipped the netcord and dropped over for a winner. Again, she clenched her fist and said "Yes!" before holding up her hand and saying "Sorry" in keeping with tennis etiquette.

Williams went ahead 5-0 in the third set but squandered three set points before serving out the match in the seventh game.

Also advancing were sixth-seeded Francesca Schiavone, who downed Barbora Zahlavova Strycova 7-5, 6-3; No. 12 Svetlana Kuznetsova, who beat Alexandra Dulgheru of Romania 6-0, 6-2; and 18th-seeded and former No. 1-ranked Ana Ivanovic, a 6-3, 6-0 winner over Eleni Daniilidou.

Day 4 started in bright sunshine, although showers were in the forecast. The tournament was about a dozen matches behind schedule because of rain delays over the first three days.

Anticipating possible rain, organizers closed the Centre Court roof for fifth-seeded Robin Soderling's match against 2002 champion Lleyton Hewitt. They were to be followed by French Open champion Li Na of China against Sabine Lisicki and six-time winner Roger Federer against Adrian Mannarino of France.

The Court 1 schedule featured matches involving Novak Djokovic, Jo-Wilfried Tsonga and 2004 champion Maria Sharapova.

Top-ranked Caroline Wozniacki is scheduled to play on Court 2 against Virginie Razzano.