HALLE, Germany — Roger Federer struggled in an "awkward" match Thursday before advancing to the Gerry Weber Open quarterfinals with a 7-5, 6-3 win against Jan Vacek.

Federer faced surprisingly strong resistance from the 342nd-ranked Czech qualifier. The top-ranked player needed a crosscourt volley to break serve for a decisive 4-3 lead in the second set.

"It was difficult," Federer said. "When it happens — you can't play your game and it isn't working — then it's a surprise. It was an awkward sort of match."

Federer ran his grasscourt win streak to 56 matches, including 22 at Halle, dating back to 2003. Federer is a four-time winner at Halle, and each time has gone on to win Wimbledon.

Federer's win came against a player with win on the ATP tour this season — in the first round earlier this week against Scott Lipsky. The 32-year-old Vacek usually plays on the lower-level Challenger tour.

Federer said a tough match was as important as an easy one, such as his first-round rout of Michael Berrer, which lasted just more than an hour.

"Now I've played a righty and a lefty," Federer said. "On the easy wins you feel good and don't think about it. On the hard ones you think about what you have to do better."

Michael Llodra also reached the quarterfinals by defeating Steve Darcis 7-6 (6) 7-6 (3).