By Steve Keating

NEW YORK (Reuters) - Former world number one Caroline Wozniacki reached the semi-finals of the U.S. Open on Tuesday with a ruthless 6-0 6-2 win over wounded Latvian Anastasija Sevastova.

Sevastova, the first Latvian to reach a grand slam quarter-final, saw her U.S. Open hopes come to a painful end when she rolled her ankle chasing down a ball on Wozniacki's opening serve.

"For sure it was affecting my play but I'm not a person that likes to retire during a match, so I just tried my best," said the 48th ranked Sevastova, who fought bravely to the finish.

"It's tough to end like that, but it's tennis. I could have played amazing but still lost. It would hurt more, I think."

After watching Sevastova crash to the court and then get up, Wozniacki said she had taken no chances with a spot in the semi-finals on the line.

With her opponent hobbled, the Dane showed no mercy as she moved in for the kill, keeping Sevastova running from sideline to sideline.

"I've rolled my ankle several times so I feel real sorry for her," said Wozniacki, twice a U.S. Open finalist. "But I was just extra focused.

"She stood up and I knew if she can still walk and still put weight on it and stuff then she's going to go obviously more for her shots and stuff like that.

"I kept serving well and made her run."

While there was no room for sympathy on Arthur Ashe Stadium court, Wozniacki knows the damage a bad ankle can cause having been sidelined herself for three months this season.

The 26-year-old was forced to withdraw from the French Open due to a sore ankle and then had a first round exit at Wimbledon, sinking her world ranking to 74 by the time she arrived at Flushing Meadows.

But for Wozniacki, ranking like age, is just a number.

"Being 70-something doesn't really mean much to me," shrugged the Dane, who next meets second seed Angelique Kerber, a 7-5 6-0 winner over Roberta Vinci.

"I still believe and feel like I'm one of the top players and grinding my way back, so that's why I'm saying for me right now the ranking is just a number.

"There is a long way for me right now to get back to number one but I'm doing my best to just play my best tennis and have fun with it.

"It's really all that I can do right now."

(Editing by Larry Fine / Ian Ransom)