BOSTON — MONICA SELES'S presence adds special flavor to the 1995 US Open Tennis Championships, which begin Monday at Flushing Meadows, N.Y. Seles seeks to continue her fairy-tale comeback from seclusion. Meanwhile, Steffi Graf, beset by personal misfortunes, hopes to redeem her status. The joint top seeds will aim for sole possession of the crown they now share. (Seles's No. 1 ranking was the result of a decision by the top 20 women players.) In the men's draw, defending champion Andre Agassi and No. 2 Pete Sampras will resume their intense rivalry, which refreshingly has not interfered with the strong off-court bond they share and cherish. Three years ago, the Graf-Seles rivalry was lively. But just when Seles seemed settled at the top, she was attacked by a knife-wielding fan at a tournament in Germany in 1993. When a German court set her assailant free, an infuriated Seles opted for exile from the tennis courts. Twenty-seven months later, Seles is back - a little taller, a bit bigger, but definitely better - armed with a potent mixture of powerful first serves and heavier ground strokes. ''Monica's return is good for the game, but it is more important to Monica than anyone else,'' said Chris Evert at her enshrinement into the Tennis Hall of Fame recently. ''It proves that she's overcome adversity.... The crowds will begin to embrace her.'' Seles celebrated her return to competitive tennis by winning the Canadian Open title in Toronto last week, losing just 14 games in the process, an accomplishment with few parallels. She opens against Ruxandra Dragomir of Romania and could meet No. 4 Jana Novatna, if the latter plays as expected. While the US Open draw lays a rosy path for Seles, Graf's route is fraught with peril. She meets Amanda Coetzer in the first round, the same woman who defeated her in the opening game in Toronto less than two weeks ago. Also in Graf's half are the talented French women, No. 16 Nathalie Tauzait and sixth-seeded Mary Pierce. Graf's biggest threat is reigning champion Arantxa Sanchez of Spain, who is capable of avenging her Wimbledon defeat and preempting a Graf-Seles showdown. ''Fraulein Forehand's'' other concerns include a recurring physical problem and the arrest of her father and mentor, Peter Graf, now in jail on charges of tax evasion. Even though American Andre Agassi cannot afford to slack off, the men's draw appears to be more or less balanced. Last year, Agassi became the only unseeded player in the Open era to win the US Open men's singles title. Racing to catch him is fellow American Pete Sampras. Among other men capable of spoiling a Sampras-Agassi final are Yevgeny Kafelnikov, Michael Chang, Thomas Muster, Michael Stich, and Richard Krajicek. All along the way, unsung predators await to add entertaining twists as they pursue fame and a share of the $9.8 million fortune in prize money. The Open will be televised on USA Cable (check local listings) and on CBS (all times EDT): Sat., Sept. 2, 12 noon-6 p.m.; Sun., Sept. 3, 12:30 p.m.-6 p.m.; Mon., Sept. 4, 12:30 p.m.-6 p.m.; Fri., Sept. 8 (women's semifinals), 11 a.m.-5 p.m.; Sat., Sept. 9 (women's singles final and men's singles semifinals), 11 a.m.-7 p.m.; Sunday, Sept. 10 (men's singles final), 4 p.m.-7 p.m.