ROME, April 19 (Reuters) - Inter Milan will be determined to prove the midweek loss to Roma that delayed their title celebrations was only a blip by beating Siena on Sunday.The Serie A leaders' defeat - their first since going down 2-0 to Bayern Munich in the Champions League group stage in September - ended their hopes of an unbeaten league season.But with a 13-point advantage over second-placed Roma, Roberto Mancini's side need only six points from their final six games to wrap up the 15th scudetto in the club's history.They could clinch the title on Sunday if they win at Siena and Roma lose away to Atalanta.'We were like a tennis player who goes to play match point and suddenly feels frightened of winning,' said Mancini after the defeat.'We played as though the handbrake was on - not just one player, but all of them - and Roma took advantage because they are a strong side.'Last year Inter were awarded the scudetto in July after the season was over when a sports tribunal revoked Juventus's title win in the fallout from the country's match-fixing scandal.Inter's Dejan Stankovic said the expectations of the San Siro crowd, who hoped to see the team lift the trophy on the pitch for the first time since 1989, affected the performance.'There was a lot of tension and we felt it. Playing for the title in front of our fans made us feel like we had weights on our legs,' said the midfielder after Wednesday's defeat.He denied there was any chance of Inter throwing the title away. 'The celebrations are only postponed. We're certainly not going to dwell too much on this defeat,' he said.Inter's form has done little to steady their fans' nerves. Having looked invincible for much of the season, they have appeared oddly fragile in their last three games.Defeat to Roma followed draws against Reggina and Palermo leading some to suggest the players are suffering burn-out.Mancini, however, insisted the problem was in the players' heads, not their legs.'They went into the match (against Roma) filled with the stuff people have been saying in the last few days - that the title was already won, that we had a 16-point lead...'If our lead had only been six points, maybe the match would have finished quite differently,' he said.Roma have scored seven goals in their last two Serie A matches and are expected to keep up the pressure on Inter with a win against Atalanta.Siena are unlikely to provide a serious obstacle to Inter's drive for the title, however.Meanwhile, third-placed Lazio face a difficult home match against Fiorentina, who are pushing for a UEFA Cup place.Neither side is in great form.Lazio have drawn their last two league games against relegation-threatened sides Ascoli and Chievo Verona, while Fiorentina's 11-game unbeaten run ended on Wednesday, when they slumped to a 2-0 defeat at Parma.If Lazio lose, they could find fourth-placed AC Milan breathing down their necks. The Serie A giants, who host Cagliari on Saturday, are in their best shape of the season and hit five goals past Ascoli in midweek.However, several of Milan's leading players, includign Kaka, are likely to be rested ahead of next Tuesday's Champions League semi-final against Manchester United.'The win is a good sign ahead of our trip to play United,' said Ancelotti. 'We've got to continue on this road and save our energy. Kaka, for example, will be rested against Cagliari'.