Vera Zvonareva struggled once again at the U.S. Open but the second seed managed to defeat pesky Spaniard Anabel Medina Garrigues 6-4 7-5 Friday and move into the fourth round of the year's final grand slam.

Zvonareva, a finalist 12 months ago, needed two hours to defeat Garrigues for the fourth time in five career meetings.

I was playing some good tennis and then there were a few moments where I felt like I disappeared, conceded the Russian, who committed 35 unforced errors while producing only 25 winners under sunny skies in Arthur Ashe Stadium.

Zvonareva, who was not at her best while defeating unseeded Ukrainian Kateryna Bondarenko 7-5 3-6 6-3 in her second-round match, will now face either unseeded American Irina Falconi or number 22 Sabine Lisicki of Germany.

A veteran of nine U.S. Opens, the 26-year-old Zvonareva said she is looking to eliminate the periods where she sprays the ball around the court.

If we take all those down moments away, I was able to raise the level of my game a little bit more today, said Zvonareva, who lost to Kim Clijsters in the 2010 final.

Once you go deeper and deeper (into the tournament), you want that very good level and you want to improve every day.

Medina Garrigues said Zvonareva's play has been spotty but added she expected the world's number two ranked player to return to form during the U.S. Open fortnight.

I don't think she's in her best moment, she said. But during the tournament, every day that passes, when she plays more matches she's going to feel better.

She's a very tough player and she can be right back up there at the end.

While admitting she needs to become more consistent, Zvonareva said she plays well when it matters the most.

When there are those tight moments, I really feel like that's where I'm able to produce my best tennis, she said. That's where I'm able to really stay focused.

It's something that comes naturally. It helps me a lot to win a lot of matches.