LONDON (AP) — In 2011, Novak Djokovic figured out the grass at Wimbledon and found himself holding the trophy at the All England Club for the first time.

This year, by his own estimate, "I'm actually playing better tennis on grass than I played two years ago."

The top-seeded Djokovic defeated No. 13 Tommy Haas 6-1, 6-4, 7-6 (4) on Monday to advance to his 17th straight Grand Slam quarterfinal. He still hasn't lost a set in the tournament.

"I feel good about myself in this moment," Djokovic said. "I'm No. 1 in the world. I have no reason to be concerned about my game."

Despite falling behind a break in the second set, then enduring a few tense moments in the third, the Serb was never really in trouble as he moved a step closer to his seventh Grand Slam title.

In the second set, he got broken to fall behind 3-2, then lost one more game before winning the final four to close it out.

In the third set, he failed to serve out the match at 5-3 and wasted a match point in the next game before closing out the tiebreaker with a forehand winner on his fourth match point. He has yet to drop a set in this tournament.

"It was a great match from my side," Djokovic said. "Minor setback in the third set. I allowed him an opportunity to come back to the match. But a great tiebreak and I'm really glad to finish in three."

On Wednesday, Djokovic faces No. 7 Tomas Berdych, who reached his first Wimbledon quarterfinal since he was the runner-up in 2010 by beating Bernard Tomic 7-6 (4), 6-7 (5), 6-4, 6-4.

Djokovic's quarterfinal streak is the longest active streak and third longest in the Open era behind Roger Federer's 36 and Jimmy Connors' 27 straight appearances in the final eight. Federer's streak ended when he was upset in the second round last week.

Federer's loss, combined with Rafael Nadal's first-round departure and a number of other upsets, created a wide-open feel on the side opposite Djokovic's in the draw. No. 2 Andy Murray is the only player on that side left among the top 20 seeds.

But on Djokovic's side, all four players seeded to be in the quarterfinals have made it: The winner of Djokovic-Berdych will play the winner of the quarterfinal between No. 4 David Ferrer and No. 8 Juan Martin del Potro, the 2009 U.S. Open champion.

Struggling with a sore big toe, Ferrer beat Ivan Dodig, 6-7 (3), 7-6 (6), 6-1, 6-1, while del Potro overcame a painful left knee, which he hyperextended on Saturday, to top 23rd-seeded Andreas Seppi, 6-4, 7-6 (2), 6-3.

"I think in these kind of matches, in quarterfinals, all the players feel something," del Potro said.

If Djokovic is feeling anything, it isn't showing. He said his history against Haas — he was 0-2 against the German on grass — made him a little nervous coming into the match.

"I just wanted to kind of focus on my game and what I want to do," Djokovic said. "I started extremely well. That was obviously helping me to feel confident on the court."