Roscoe Tanner, whose intimidating serve and long and happy experience on hard courts made him one of the favorites to challenge top seeds Bjorn Borg and Jimmy Connors in the U.S. Open tennis champion-ships, saved a matched point in a thrilling final set tie breaker and beat Victor Pecci of Paraguay, 6-3, 4-6, 7-6, to advance to the thirdround yesterday.

Tanner 27-year-old left-hander from Lookout Mountain, Tenn., saved a break point as he served 4-4 in the final set, and then a match point at 5-6 in the best-of-12-point tie breaker that brought a standing ovation from the crowd of 6,000 jammed around the granstand court at the National Tennis Center.

Pecci - a 22-year-old, 6-foot-3 Paraguayan with a serve almost as oppressive as Tanner's - blandeted the net with his long reach and put up a magnificent fight, but his backhand return of serve deserted him when he needed it most.

At 5-5 in the decisive tie breaker, Pecci received an apparent bad call, the baselineman calling a blistering forehand passing shot by Tanner good. It appeared to be long. Pecci, distraught, appealed to the chair umpire, who over-ruled the call.

The reversal made it 6-5, Pecci: match point against Tanner. He went for an ace down the middle, but missed his first serve. Then he took a deep breath and hit a deep second serve to the backhand that Pecci knocked into the net.

A stretching forehand down-the-line valley brought Tanner to match point for the first time. Pecci get in his first serve, but Tanner made a good backhand return to force a backhand volley error to end it.

The No. 7 seed, Italian Corrado Barazzutti, was beaten yesterday afternoon. A clay court specialist who loathes the hard surface courts here, Barazzutti fell to big-serving John Sadri, 21, of North Carolina State University, who turned pro this summer after reaching the final of the NCAA Championships. The scores were 7-5, 4-6, 6-1.

Martina Navratilova, the Wimbledon champion who is seeded No. 1 in her quest for her first U.S. title, easily disposed of Gail Lovera of France, 6-1, 6-1.

JoAnne Russell, the NO. 14 women's seed, was casted by talented Czechoslovakian junior Hana Mandlikova, 6-2, 4-6, 6-2.