Juan Martin del Potro will return to tennis after a month away by defending his Estoril Open title from Monday.
The Argentine has not competed since the Davis Cup quarter-final win over Croatia at home where he won both of his singles rubbers.
But the world number 10 will quickly need to shake the rust out of his game with just four weeks left until the start of the French Open on May 27.
Top seed Del Potro heads the field at the ATP-WTA event at the Estadio Nacional which includes Frenchman Richard Gasquet, Swiss Stanislas Wawrinka and Spaniard Albert Ramos.
The WTA field is led by Italian Roberta Vinci, 2008 winner Maria Kirilenko of Russia, Spain's Anabel Medina Garrigues and Czech Petra Cetkovska.
The top four seeds in the men's event have first round byes while the leading women go straight into action on the clay which should play heavy after continuing rain and more forecast for the week.
Del Potro will stand as the main drawcard for the week in Portugal after the injury withdrawal of Frenchman Gael Monfils, still unfit after pulling out of the Davis Cup at the start of the month with an abdominal injury.
Second seed Gasquet is also coming back from fitness problems which forced him to miss the Monte Carlo Masters, delaying the start of his Roland Garros preparations.
Del Potro beat Spain's Fernando Verdasco in the 2011 final.
Juan Martin del Potro (ARG x1), Richard Gasquet (FRA x2), Stanislas Wawrinka (SUI x3), Albert Ramos (ESP x4), Denis Istomin (UZB x5), Robin Haase (NED x6), Alberto Montanes (ESP x7), Flavio Cipolla (ITA x8)
Roberta Vinci (ITA x1), Maria Kirilenko (RUS x2), Anabel Medina Garrigues (ESP x3), Petra Cetkovska (CZE x4), Zheng Jie (CHN x5), Kaia Kanepi (EST x6), Nadia Petrova (RUS x7), Mona Barthel (GER x8)