Federer was due to play in the 2-9 May Madrid Open, but made a late decision to switch to the lower-profile Geneva event where he should have a better chance of getting on a roll.
Williams is preparing for the French Open, which starts at the end of the month.
Andy Murray has been out of action since suffering groin pain ahead of the Miami Open in March.
Nadal is the top seed in Madrid and will play either compatriot Carlos Alcaraz or the Frenchman Adrian Mannarino in the first round while he is due to meet Dominic Thiem, a finalist at the French Open in 2018 and 2019, in the last four.
As part of the progressive relaxation of restrictions, the "limit will be 35% of the capacity of the courts with a maximum of 1,000 people per court," said Blanquer.
The Australian will be one of the favourites at Roland Garros after her victory there in 2019 and because of her excellent recent form.
The Australian rallied from losing the first set to claim a 3-6, 6-0, 6-3 win over Sabalenka, who needed a medical time-out after the second set for an adductor injury.
Rafael Nadal beats Pablo Carreno Busta to storm into final at Barcelona; seeks Stefanos Tsitsipas revenge
Top seed Nadal, an 11-time champion in Barcelona, eased to a 6-3, 6-2 win over World No 13 Carreno Busta in 90 minutes.
French Open 2021: Ashleigh Barty doesn't consider herself as defending champion, says 'it's a fresh start for everyone'
The Australian won her only major singles title in Paris in 2019. The tournament was cancelled last year because of the Covid-19 pandemic.
Federer, who will turn 40 in August, has played just one tournament in 2021, winning one match in Qatar in March on his return from 13 months on the sidelines after undergoing two bouts of knee surgeries.
Rafa Nadal is aiming for a 12th Masters title in the principality ahead of his pursuit of a 14th French Open success and a record 21st Grand Slam.
Nadal has 71-5 record in Monte Carlo where he has fallen short of the quarter-finals just once - on his debut in 2003 as a 16-year-old.
The French Tennis Federation said it hoped the delay could enable more fans to attend if COVID-19 restrictions are eased by then.
Last year's French Open was delayed by four months and held in September and October instead of its longstanding May-June slot.
Federer hasn't won a tour clay title since Istanbul in 2015; his most recent Masters triumph on the surface was Madrid in 2012.
Discussions between the ministry and Roland Garros officials also included how the tournament might be able to accommodate fans who were largely locked out of the delayed 2020 edition. This year's event is due to take place between 23 May and 6 June.
Barty, the 2019 French Open champion, has held the No 1 ranking since September of that year, despite skipping the rescheduled US and French Opens in 2020 to remain in Australia.
The 35-year-old will miss clay court events in Marbella, Monte Carlo and Belgrade in April.
Last year's tournament was switched unilaterally from spring to September-October as the coronavirus sent the sport into a five-month suspension.
Paes, 47, said he skipped the Australian Open as he is apprehensive of playing in a bio-bubble but is looking ahead to the French Open.