MIAMI, FL, USA -- No.13 seed Sloane Stephens of the United States had to fight off a spirited effort from Romanian qualifier Monica Niculescu at the Miami Open on Saturday night, surviving through two challenging sets before Niculescu had to retire due to an injury to her mid-back during the third set. The final scoreline was 6-7(1), 6-3, 4-0, ret.

“I was just playing good, battling well," Stephens said, after the match. "She’s a really tough opponent to play, so I had to stay focused. Obviously it’s unfortunate that it ended like that, but I was really pleased about how I played the whole match, so that’s good.”

Stephens held a 4-1 lead in the opening set before the Romanian charged back to win it in a tiebreak, but the American refocused to claim a difficult second set en route to victory. Stephens has now won three consecutive matches against Niculescu after losing their first encounter in 2011.

Reigning US Open champion Stephens enters the fourth round in Miami, where she will play No.3 seed and reigning Wimbledon champion Garbiñe Muguruza of Spain, who quickly dispatched American Christina McHale, 6-2, 6-1, in the nightcap match.

“I think it’ll be a good match," said Stephens. "I haven’t played [Muguruza] in a long time, so I’m looking forward to it.”

Niculescu started the match by putting pressure on the Stephens serve, holding three break points in the American’s first two service games. But Stephens fended those off and ended up with the first advantage, going up a break at 3-1 after Niculescu double faulted down break point.

However, Niculescu got back on serve at 4-3, frustrating Stephens with her patented slice forehand and manipulating the American into unforced errors. Stephens had to face break points in her next two service games -- she faced ten overall in the set, saving eight -- and was broken for 5-5, giving Niculescu a chance to serve out the set.

But the Romanian could not close it out, with Stephens reaching break point via a winning volley, and then breaking for 6-6 after Niculescu made a drop shot that failed to clear the net. After a tightly contested hour-long set, the players advanced to the tiebreak.

Niculescu won a delightful, all-court first point of the tiebreak with a reflex volley, and went on a tear from there, taking the first four points in total. Stephens got on the board for 4-1 before more unforced errors followed from the American, giving Niculescu five set points. Stephens ended the tiebreak with a double fault, handing Niculescu the opening frame.

The second set opened with easy holds by both players, but that did not last, as Stephens and Niculescu would face challenges on serve. With Niculescu serving at 1-1, Stephens reached triple break point, but the Romanian saved all three before holding, after a backhand error by the American. Niculescu then had two break points of her own in the next game, but Stephens dismissed them before holding with a forehand winner.

The pair then exchanged breaks, before Stephens obtained what would become the decisive break of the match at 3-3, claiming the lead off of a forehand winner. The American then had to survive yet another lengthy game on her serve, repelling one break point before Niculescu's unforced errors started to pile up, allowing Stephens to hold for 5-3.

By this point, Stephens was demonstrating the variety of shot which brought her to her maiden Grand Slam title last summer, and she broke Niculescu an additional time to claim the second set, and tie the match at one set apiece.

After Stephens held in the opening game of the deciding set, Niculescu called for the trainer, who came to the court to work on the Romanian's back. A clearly uncomfortable Niculescu attempted to soldier on, and staved off two of three break points in the next game before Stephens took the 2-0 lead behind a crosscourt forehand winner.

Niculescu kept fighting, even reaching break point in the third game of the set, but misfired on her service returns at crunch time, letting Stephens hold for 3-0. After Stephens broke once more for 4-0 behind a backhand volley, Niculescu's injury was too much for her to contend with, and the Romanian retired.