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 on 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." 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 beating 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."
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 183 kph (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 back out and held serve, with Williams yanking a backhand wide on set point.
With her back to the wall, 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 hit 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 forecast for later. The tournament was about a dozen matches behind schedule due to 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 was due on Court 2 against Virginie Razzano.