Vasek Pospisil eliminated top seed Tomas Berdych at the Citi Open on Thursday, winning 6-2, 6-4 to progress to the quarterfinals.

Berdych unofficially committed 21 unforced errors and had his serve broken four times, including the final game of each set. Two of Pospisil's three career top-10 wins have come against Berdych, the first last year in Montreal.

Berdych trailed 0-3 in the first set and never recovered. Pospisil, ranked 36th, only lost serve once, fighting back three of four break points.

"The beginning was very key to the whole match," said Berdych, who said his practice time had been reduced by a back injury.

The Czech noted that Pospisil -- who won the Wimbledon doubles title with American Jack Sock last month -- was a player who does his best in big matches.

"I prefer center courts. I just prefer playing in front of people and on bigger stages. ...It's motivating," said Pospisil.

Pospisil only lost serve once, fighting back three of four break points. He will next face No. 10 Santiago Giraldo, a 6-2, 6-0 winner over Victor Estrella Burgos.

American Steve Johnson knocked off another experienced seed, beating No. 9 Ivo Karlovic 3-6, 7-6 (4), 7-6 (7).

The American will face seventh-ranked and second-seeded Milos Raonic, the Canadian who edged Australian veteran Lleyton Hewitt 7-6 (1), 7-6 (3).

Johnson, ranked 68th, has had 56 aces served against him over the past two matches, against Karlovic and American compatriot John Isner, yet managed to beat both.

Against Karlovic, Johnson fended off two match points in the third-set tiebreaker and went up 8-7 on the Croat's third double fault of the match.

Next up, 6-foot-5 Raonic. The Wimbledon semifinalist ranks third in aces behind Isner and Karlovic.

"They're getting shorter every day," Johnson cracked.

Sixth-seeded Richard Gasquet beat Tim Smyczek 6-3, 6-2. Seventh-seeded Kevin Anderson also advanced, beating Malek Jaziri 6-3, 6-4. Donald Young beat No. 15 Denis Istomin 6-3, 3-6, 6-3 for his second straight victory over a seeded player.

In women's second-round play, second-seeded Ekaterina Makarova beat Lauren Davis 6-2, 6-4; fifth-seeded Anastasia Pavlyuchenkova topped Hiroko Kuwata 6-3, 6-3; sixth-seeded Svetlana Kuznetsova beat Kirsten Flipkens 6-4, 7-5; and Vania King routed Christina McHale 6-1, 6-3.