The 16-time grand slam champion is making a rare visit to the ATP 250-level for a specific reason: lifting his clay game in time for the May 23 start of his French Open title defence.
The number one made the call last autumn to return to Estoril and is sticking to his plan to play the tournament where he won the 2008 title when Russian Nikolay Davydenko had to quit injured in the final.
The Swiss is also sure to have mentally erased his second-round upset loss to Ernests Gulbis at Rome, with his current week's goal nothing less than a second title in Portugal.
"When you lose, you actually realise how tough it is to dominate this knock-out system we have in tennis which is the rule," said Federer, whose only title so far this season came at the Australian Open.
"One week you're great and the next week you are terrible."
Federer begins against either a qualifier or German Bjorn Phau.
The format will be kind to Federer with the top four seeds receiving byes in the 32-man draw. The possibility of playing a maximum of four matches especially appealed to Federer, who is defending a Masters title at Madrid the following week.
French former ATP number nine Gael Monfils will take a wild card, returning to court for the first time in a month and a half after a hand injury.
Monfils, seeded third, played at the Estadio Nacional in 2006 and 2007 and stands 1-2 at the venue. He also receives a bye and could first face a pair of qualifiers.
The number 18 will step onto clay for the first time in 11 months, when he reached the Roland Garros quarter-finals and lost to Federer.
Ivan Ljubicic, seeded second, returns to Portugal after an absence of a decade after losing the only match he played at the event to Nicolas Lapentti.
The Croatian's last big effort came in march when he won the Indian Wells Masters, but he stands 3-3 since that final.
In the simultaneous women's WTA event, 35th-ranked Hungarian Agnes Szavay heads the field with Romanian Sorana Cristea expected to challenge.
Roger Federer (SUI x1)
Ivan Ljubicic (CRO x2)
Gael Monfils (FRA x3)
Albert Montanes (ESP x4)
Guillermo Garcia-Lopez (ESP x5)
Florian Mayer (GER x6)
Juan Chela (ARG x7)
Pablo Cuevas (URU x8)