MIAMI, FL, USA -- A highly competitive and intriguing match on Thursday night ended abruptly, as wild card Victoria Azarenka of Belarus came through her second-round encounter with No.14 seed Madison Keys of the United States, 7-6(5), 2-0, ret. Keys retired due to a left thigh injury.

"It's the worst way to finish the match," Azarenka said, during her post-match press conference. "I always feel bad for the opponent. But it is sport, and these things happen. The most you can hope is Madison recovers as soon as possible. Hopefully she will be able to play next tournament, and, you know, it won't affect the rest of the season."

Azarenka survived a topsy-turvy opening set with multiple lead changes and streaks of solid tennis by each player, barely clinching a first-set tiebreak containing more mercurial play by the competitors, before Keys had to retire after the second game of the second set.

"I think I raised my level compared to Indian Wells pretty dramatically," Azarenka continued. "Matches like this are very important, in the first set to be down and still be able to find a way to win. Those are the things that brings confidence, brings more, you know, the rhythm, and the competition feel."

It was the first meeting between Keys, the 2017 US Open finalist, and former World No.1 Azarenka, and the extremely close opening set took 66 minutes to complete before Keys called out the trainer between sets. After 14 more minutes, the American stopped the match after falling behind a break in the second set.

Both players had more unforced errors than winners: 9 winners to 21 miscues for Azarenka, and a ratio of 14 to 25 for Keys. Azarenka, though, was more successful on her first serve, winning 72 percent of those points, as opposed to a success rate of 55 percent on the first delivery of Keys.

Azarenka leapt out to an early 3-1 lead in the match, as the Belarusian exhibited the smothering aggression which led to her dismantling of another American, CiCi Bellis, in the opening round yesterday.

Keys, however, utilized a pep talk from her coach on the following changeover to focus her power game. Holding a break point on Azarenka’s serve at 3-2, the American rocketed a forehand to force an error from Azarenka, leveling the match at 3-3.

After a quick hold for 4-3, Keys continued to apply pressure to the inconsistent Azarenka second serve. Azarenka saved three break points early in that game, but the Belarusian double faulted on a fourth break point to hand Keys a 5-3 lead and an opportunity to serve for a one-set advantage.

"I felt like I started really well and I started to dominate," Azarenka explained. "Then I kind of lost my lead and let her into the match and let her dictate a little bit the points. I made a couple unforced errors where I shouldn't have, so I let her kind of get back into that first set."

Like the rest of the set, though, expectations were subverted and leads were tenuous at best. Azarenka’s returns improved while the groundstrokes of Keys started to miss their targets, and Azarenka broke Keys to get back on serve at 5-4, ending the American’s four-game winning streak.

Keys had a final chance to swipe the set from Azarenka on the Belarusian’s serve at 6-5, as Azarenka struggled with unforced errors and multiple double faults in the game. Nevertheless, Azarenka resisted two set points in a protracted game, holding for 6-6 and sending the opening frame to a tiebreak.

The tiebreak, like the set itself, featured streaks of solid play by both players. Azarenka was hitting her forehand well en route to a seemingly insurmountable 5-0 lead, but a double fault and a backhand error allowed the American to reclaim two mini-breaks, and eventually edge to 5-3.

A forehand error by Keys at 5-3 stopped her run and gave Azarenka triple set point, but, once more, Keys took both points on Azarenka’s delivery, putting her back on serve in the tiebreak at 6-5. Keys failed to keep that momentum, though, and pushed a shocking backhand long to cede the tiebreak and the set to Azarenka.

"After she was leading, I just tried to stay focused, tried to stay tough and play one point at a time and use my opportunities," said Azarenka. "As soon as I had opportunities, I was trying to be aggressive and not waiting for her to make mistakes, because she's not going to really do that. So I really had to stay tough and take that first set."

After having her left thigh worked on and wrapped between sets, Keys made an attempt to work through the remainder of the match. Azarenka held at love to open the second set, and Keys made an effort to hang with the Belarusian. But after struggling with the injury and being broken to give Azarenka a 2-0 lead, the American unfortunately had to call it quits.

"I felt my hamstring kind of tighten up in the middle of the first set, and at the end of the first I really felt it get worse," Keys told the media in her post-match press conference. "Then playing the first couple of games, it wasn't getting better, and I didn't want to make it worse than it already was."

"I think I have made some injuries worse by trying to play through it, and I'm just not interested in doing that at this point of the season right now," Keys continued. "So I think it was just the smarter idea to get off the court."

In the third round, Azarenka will play No.20 seed Anastasija Sevastova, after the Latvian defeated Frenchwoman Alizé Cornet earlier on Thursday, 7-5, 6-4.

"The most important [thing] is that I'm still able to play and do what I love to do, because I have been doing this for 20-something years now, and I still have, you know, dreams and goals that I want to achieve," Azarenka concluded. "So that's really fun."