ROME, Italy -- Danielle Collins of the United States moved into the second round of the Internazionali BNL d'Italia when No.11 seed Caroline Wozniacki of Denmark retired due to a left lower leg injury after the American claimed the first set of their opening-round encounter, 7-6(5).

"Obviously, I feel for Caroline, what she’s going through with her medical condition and her injuries," Collins said, after the truncated encounter. "It’s never fun to go out there and play against somebody that’s hurt, and dealing with that kind of pain. I was happy with the way that I played, but also, it’s just not a fun situation."

It is the second consecutive event in which former World No.1 Wozniacki was unable to complete her first-round tilt, following an opening-round injury retirement to Alizé Cornet at the Mutua Madrid Open last week.

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Nevertheless, it was Collins who claimed her fourth Top 20 victory of the season, the other three coming during her breakthrough semifinal run at the Australian Open. The American grinded out a 61-minute first set, winning two-thirds of her service points and firing three aces before the early conclusion.

"I feel that I’m adjusting to the clay pretty well," said Collins. "I had a great result in Charleston [reaching the quarterfinals], and played some great tennis in Madrid and just kind of fell a little short. I’m comfortable on this surface, and making improvements within my game and doing a little bit better than I did last year, so it’s exciting."

Collins finds herself into the second round at Rome for the second straight year. Up next for the World No.30 is another former World No.1, Garbiñe Muguruza of Spain. Muguruza claimed her spot in the second round on Monday with a straight-set win over Zheng Saisai of China. It will be the first meeting between the two.

"I’m just looking forward to hopefully carrying over some good tennis into the next match, and have a great match against Muguruza," Collins stated. "She does a lot of great things in her game that separate her from a lot of players. She hits the ball really big, she looks to take control of the points early. It’s going to be a tough match, but I’m looking forward to the challenge." 

The American got off to a slow start, dropping serve in the opening game with a plethora of errors, while Wozniacki eased to a 2-0 lead after a comfortable love service game. But Collins quickly regained the aggressive play which brought her into the final four of the Australian Open this year, cracking a forehand return winner to break back for 2-2.

Collins moved into a run of five straight games, using stunning down-the-line backhands to overwhelm Wozniacki and take control mid-set. After an ace by the American closed out a hold for 5-2, Wozniacki received her first of three visits from the physio.

Initially, the treatment seemed to help Wozniacki, as the former World No.1 reclaimed potency on her forehand wing. The Dane broke Collins when the American served for the set at 5-3, and the combatants were back on serve, where they remained up to a decisive first-set tiebreak.

Collins took command in the tiebreak with a forehand winner to garner a crucial mini-break for 4-3, and blasted two exceptional serves from there to reach triple set point at 6-3. Wozniacki held onto her two service points, but a forehand winner down the line at 6-5 sealed the set for Collins.

Wozniacki quickly shook her opponent’s hand after that point, as the Dane was too hampered to continue into further sets.