Jonas B. Svensson turned a positive attitude into a fourth-round upset of Boris Becker to power into the quarterfinals of the Australian Open.

Svensson, seeded 14th, reached the last eight of a Grand Slam tournament for only the second time in his career with a 7-6 (7-5), 6-4, 6-3 victory over the two-time Wimbledon champion today.The Swede rifled a succession of smooth groundstrokes past the increasingly frustrated West German to win in just over two hours. Svensson went out with a positive attitude and was never put under pressure.

"I knew it could be my day," said Svensson, who reached the French Open semifinals last year and has been a more confident player since. "I'm not surprised. I really thought I could win from the start."

Becker screamed in anguish and threw his racket in the second set as Svensson served splendidly and hit relentlessly accurate groundstrokes.

"When he started to scream I knew he was on the way down mentally," Svensson said. "I knew I just had to play my own game and stay out there and fight."

Svensson dropped only 15 points on serve while Becker had trouble keeping the ball in play, piling up 38 unforced errors.

"He played three sets of superb tennis and didn't give me a chance to come back into the match," said Becker, who last month led West Germany to victory over Sweden in the Davis Cup final.

Svensson, ranked No. 4 in Sweden, made the fourth round of the Australian Open last year before losing to eventual finalist Pat Cash. He now faced fellow Swede Jan Gunnarsson in the quarterfinals.

Czechoslavakia's Miloslav Mecir, ranked 13th in the world, needed just 94 minutes to defeat Christo van Rensburg of South Africa in another fourth-round match.

Mecir, seeded ninth, beat Van Rensburg, ranked 36th in the world, 6-4, 6-1, 6-0 to move into the quarterfinals against Yugoslavian teen-ager Goran Ivanisevic.

Ivanisevic, playing only the second Grand Slam tournament of his career, also reached the last eight. Ivanisevic, 17, rallied to beat Mexican Davis Cup player Leonardo Lavalle 3-6, 3-6, 6-3, 6-4, 6-1.