SAN JOSE, CA, USA -- Danielle Collins of the United States moved into the semifinals of the Mubadala Silicon Valley Classic on Friday after her opponent, former World No.1 Victoria Azarenka of Belarus, was forced to retire due to injury. Azarenka had won the first set 7-6(4), but was trailing 0-3 in the second set when she retired due to a back problem.

"I’m not entirely sure what happened, but it’s never fun winning a match like that," Collins told the press, after the match. "Hopefully [Azarenka] can get back on the horse pretty soon."

Azarenka, who won the title in 2009 when it was played in Stanford, pulled through a dramatic and closely-contested first set after 72 minutes of play, but became hampered by her back injury in the first game of the second set, at which point Collins took control of the tilt before the former World No.1 could continue no further.

"I really wanted to make the match as physical as possible because that’s one of my weapons, and that’s something I felt I could really expose on her," said Collins.

"I just had to keep my foot on the gas: even if there were times when I lost a long point or she came up with a really good shot, I just kind of had to keep that mentality of getting in every extra ball that I could, and just keep running after everything and making it as physical as possible," Collins elaborated.

Consequently, Collins reaches her second WTA semifinal of the year, following her final four run in Miami. She will face fellow unseeded player Maria Sakkari of Greece, who stunned No.3 seed Venus Williams in straight sets later on Friday.

"I’m really having a great year, and happy to get these wins and get more and more experience under my belt," said Collins.

When both athletes were playing their best, the first set was an engrossing encounter that could have easily gone either way. Azarenka started the match as the better player, quickly breaking Collins in the first game of the meeting and maintaining that break lead for the majority of the set.

But Collins demonstrated her steeliness early on, using her exceptional defensive skills to prolong points until she could match Azarenka with her groundstroke prowess. The American staved off five break points in a staggering 10-minute game, holding for 3-2 and preventing the Belarusian from taking a commanding 4-1, double-break lead.

Collins was rewarded for that effort when she broke Azarenka as the former champion served for the set at 5-4; the Belarusian handed over that service game after pushing her more reliable backhand long on break point. But, at 5-5, Collins could not hold for the lead, swiftly dropping serve again to give Azarenka a second chance to serve out the set.

At 6-5, Azarenka raced to triple set point, but was too tentative on her chances while Collins pressed with aggressive forehands. Azarenka squandered four set points in total in that game, eventually losing serve with a woebegone drop shot attempt into the net.

Nevertheless, in the tiebreak, Azarenka took a 3-1 lead behind powerful groundstrokes, and maneuvered her way to a fifth set point at 6-4 when a strong serve was returned wide by Collins. On the next point, Azarenka finally clinched the set when a volley forced a long error by the American.

After powering through a protracted and tight first set, it was unfortunate for Azarenka when she began to exhibit signs of severe discomfort during the very first game of the second set. Collins claimed the initial game of the set, breaking serve with a stellar lob winner over the disheartened Azarenka.

After Collins held for 2-0 with a thumping backhand winner, Azarenka stepped to the line again, and fired three double faults in the game to drop serve once more and give Collins a 3-0 advantage. Azarenka received treatment after that game, but the pain was too great for her to keep playing, and the former World No.1 made the tearful decision to abandon the match.