Mantilla defeated Dirk Dier and Àlex Corretja, eventually falling to the eventual champion Yevgeny Kafelnikov in straight sets. After this he went on to win his first ATP title in Oporto over Hernán Gumy 6–7, 6–4, 6–3. Mantilla proceeded to make two more finals in Gstaad and Umag where he lost to Albert Costa on both occasions. In total for 1996 Mantilla won three Challenger titles and his first title in Oporto and made three finals. In 1997 Mantilla went a career best of 53–22 for the season. He achieved his best result at the 1997 Australian Open before falling to finalist Carlos Moyà in the quarter finals. Mantilla made his first TMS final in Hamburg, losing easily to Andriy Medvedev. He then won all his four singles matches as Spain won the World Team Cup. After a disappointing second round exit to Magnus Larsson at Roland Garros, Mantilla went on a hot streak and won four titles in his next five tournaments at ATP level and in between made the final of the Braunschweig Challenger losing to Francisco Roig. Mantilla's run began at Bologna where defeated reigning French Open champion Gustavo Kuerten, then followed up with titles in Gstaad over Joan Albert Viloca, in Umag over Sergi Bruguera and in San Marino over Magnus Gustafsson. Mantilla won another title in Bournemouth over Carlos Moyà and finished the year ranked 16th. This year also had his best performance at the US Open reaching 4th round losing to Richard Krajicek, and in 3rd round won over John Van Lottum in 5 sets, playing last sets with big cramps that impeded normal movement. He closed the best year in his career with a victory on National Tennis Masters competition in Spain (national version of ATP World Tour Finals) defeating Francisco Clavet at the final in Zaragoza after losing the first set. After winning Clavet, Mantilla declared: "Next year I want to win more important tournaments, shine in the big occasions and be a better tennis player". In 1998 Mantilla after losing a bet with Luis Lobo in Rome, in which he had to dye his hair blonde. Mantilla reached his highest ever ranking of number 10 on 8 June after reaching the semi finals at Roland Garros. He beat Wolfgang Schranz, Byron Black, Fabrice Santoro and Ramón Delgado to reach the quarter final. There he defeated Thomas Muster, gaining revenge for his