In 1998, she opened with a second singles title in Gold Coast. Then she broke into the WTA top 20 and reached another semifinal in Sydney, defeating Conchita Martínez. Throughout that year, Sugiyama showed consistency: a third WTA Tour title at the Japan Open, quarterfinals in Tokyo (Pan Pacific), Berlin, defeating No. 4, Amanda Coetzer, playing Strasbourg and San Diego, defeating Steffi Graf, Luxembourg, third round in Indian Wells, Miami and Montreal and second round at both French and US Open. In 1999, she reached the final at the Japan Open, the semifinals in Gold Coast and Tokyo (Princess Cup) defeating No. 8, Julie Halard-Decugis, the quarterfinals in Strasbourg and Moscow, defeating No. 6, Mary Pierce, 3rd in Indian Wells, Montreal, defeating No. 7, Jana Novotná, and at the US Open. She also reached the second round at the French Open and Wimbledon. The same year, Sugiyama won the US Open mixed doubles with Mahesh Bhupathi, her first Grand Slam title.