Stockholm: Two-time champion James Blake rediscovered the best of his past form with a 6-4, 6-4 upset of second seed Juan Martin del Potro on Wednesday to reach the quarter-finals of the Stockholm Open.

The result gave the 69th-ranked American, the champion in 2005 and 2006, his first quarter-final since making the last eight in the Swedish capital a year ago.

Due to several seasons of injuries, the 31-year-old Blake has spent several weeks this season playing Challenger events, winning two of the lower-level titles and the American said he's not felt this good in years.

"This is my best win of last three years," said Blake. "This is the best I've felt on court for a long time. To get a win like this against a top player is great.

"I've not done this for four or five years. I'll be proud of this one for a long time.

"The way I played today was what it felt like when I won here. Back then things came a little easier and the body recovered quicker. But I'll take it easy tomorrow, I think my game and my experience has helped me."

Del Potro was competing this week for the first time in a month as he builds for the Davis Cup final against Spain in December.

"He played better than me today — it's over for this week," said Del Potro. "I have a few days now to be ready for next event. I must work very hard."

Blake won the first set in 49 minutes with a stinging reflex return at Del Potro's feet on set point and secured victory with a break in the final game of the second set with a volley winner into the open court.

Next opponent

The American could face Argentine David Nalbandian in the quarter-finals if the South American beats Croatian Ivan Dodig.

South African fifth seed Kevin Anderson moved into the next round with a 6-7 (3/7), 6-4, 6-4 comeback win over German Tommy Haas.

Anderson broke once in each of the last two sets and fired 21 aces and 55 winners to move into a match against either top seed Gael Monfils or Australian Bernard Tomic.

"I'm really, really happy to get through. My serve was the biggest positive today," said Anderson.

"I served much better than my first match. It was tough to break Tommy but I managed one in each set."

Tomic tamed Estonian qualifier Jurgen Zopp 7-6 (7/5), 4-6, 6-0 to set-up a date with Monfils, who had a first-round bye and is making his Stockholm debut.

Monfils, still theoretically in the chase for a spot in next month's eight-man World Tour Finals in London, will be testing a knee injury which forced him to miss last week's Shanghai Masters, where Tomic earned a top ten win over American Mardy Fish.

RESULTS

Stockholm Open:

Men (second round, prefix denotes seeding): James Blake (US) beat 2-Juan Martin Del Potro (Argentina) 6-4 6-4; 6-Milos Raonic (Canada) beat Philipp Petzschner (Germany) 6-3 6-3; 5-Kevin Anderson (South Africa) beat Tommy Haas (Germany) 6-7(3) 6-4 6-4.

First round: Sebastian Rieschick (Germany) beat Michael Ryderstedt (Sweden) 6-4 3-6 6-3; Bernard Tomic (Australia) beat Juergen Zopp (Estonia) 7-6(5) 4-6 6-0; Grigor Dimitrov (Bulgaria) beat Ryan Sweeting (US) 6-3 6-1.