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.
Federer hasn't won a tour clay title since Istanbul in 2015; his most recent Masters triumph on the surface was Madrid in 2012.
In sum, no 20-something appears poised to wrest the mantle from the 30-somethings who have dominated the sport for so long.
The 23-time Grand Slam champion joins Roger Federer, Rafael Nadal and Novak Djokovic among the stars to have pulled out of the tournament, which gets under way on Monday with qualifying rounds.
Tsitsipas, who was coming off semi-final appearances at the Australian Open and Rotterdam, was denied a sixth ATP trophy.
The 33-year-old Serbian star won his 18th Grand Slam singles crown and ninth Australian Open singles title last month.
Their clash in the Gulf desert nation was characterised by hot, dusty wind that slowed down their exchanges but world number 42 Basilashvili bided his time to snatch the first set in a tiebreak.
Federer underwent two knee surgeries last year and was playing his first tournament since a semi-final loss to Novak Djokovic at the Australian Open in January 2020.
Looking ahead, Federer also said that he had not taken decisions on tournaments before Wimbledon or the Tokyo Olympics but hinted that he would likely be at Roland Garros.
Qatar Open: Roger Federer squanders match point in second match after return, knocked out by Nikoloz Basilashvili
The 20-time Grand Slam title winner, who underwent two knee surgeries in 2020, secured the first set but eventually wilted and lost the quarter-final tie, 3-6, 6-1, 7-5.
What matters far more in the scheme of things than the result or the way it came about is that Federer cares about success and can conjure up what's necessary to find it.
"It's been a long and tough road for me," said Federer, who hasn't played a match since a semis defeat to Novak Djokovic at the Australian Open in January 2020, 405 days ago.
Federer hasn't played a match since his semis loss to Novak Djokovic at the Australian Open in January 2020 and joked with the umpire about the rules of the game at the start of his game against Dan Evans at Doha.
Roger Federer had two operations on his right knee since playing at the Australian Open last year.
'Retirement was never really on the cards,' says Roger Federer ahead of return from lengthy injury-forced break
Federer hasn't played a match since a semi-final defeat by Novak Djokovic at the Australian Open in January 2020 and acknowledged it was unusual for a player of his age to return after injury.
Novak Djokovic began his 311th week as the No 1 player in the men's rankings on Monday.
Qatar Open: Roger Federer pain-free ahead of return from injury, says retirement 'was never on the cards'
The 39-year-old hasn't played a match since a semi-final defeat to Novak Djokovic at the Australian Open in January 2020 and acknowledged it was unusual for a player of his age to return after injury.
The 39-year-old hasn't played a match since a semi-final defeat to Novak Djokovic at the Australian Open in January 2020.