Aryna Sabalenka expressed her delight at overcoming a "tough" match with Victoria Azarenka to advance to the semi-finals of the Washington Open in straight sets. 

World number three Sabalenka needed an hour and 45 minutes to get past her fellow compatriot, prevailing 6-4 6-4 to progress to the next round. 

The two-time grand slam champion served eight aces compared to Azarenka's one, and although the encounter looked one-sided, it was anything but for Sabalenka. 

"Always tough battles against her," Sabalenka said. "She always brings the best fight she can.

"I always enjoy playing against her. Just super happy to get through this tough match.

"I feel like with every set, I play better and better. Hopefully I can just (keep) building my level and by the US Open, I'll be 100 percent ready."

Sabalenka claimed her second Australian Open title earlier this year, but was forced to withdraw from Wimbledon with a shoulder problem. 

However, the Belarusian showed no signs of injuries in the American capital and is confident of maintaining her recent form. 

"I'm super happy. I'm doing really great so far. Fingers crossed, the injury's in the past. I feel really good," Sabalenka said. 

"We're doing everything we can with my team to keep my shoulder strong and make sure (such an injury) will never happen again."

Top seed Sabalenka advanced to Saturday's semi-final where she will meet Marie Bouzkova, who beat 19-year-old American Robin Montgomery in straight sets.