But the year-end championship has been back under cover since 2005.
Federer, best able to express his manifold gifts on faster surfaces, has nonetheless been eclipsed at the O2 Arena by Novak Djokovic, who won four straight titles from 2012 to 2015, beating Federer in two of those finals in straight sets.
Djokovic is an underrated player indoors but not an underrated player on hard courts, which are now the indoor surface of choice. But even if Djokovic leads Federer, 5-4, in indoor matches, Federer still holds the overall edge. His career indoor record of 269-64 and winning percentage of 80.8 are superior to Djokovic, who is at 135-37 (78.5 percent).
Those career records are not quite complete, however, because they do not include indoor matches played at the Grand Slam tournaments. All of the slams, except the French Open, now have stadiums with retractable roofs that can be closed in inclement weather.
But with or without the Grand Slam factor, the career indoor records from this era do not quite match up with those of the best players earlier in the Open era like John McEnroe and Ivan Lendl. They played more often indoors then. There were 19 indoor events in the first year of the ATP Tour in 1990. That was down to 15 this year. Consider that Jimmy Connors won 53 career indoor titles and McEnroe won 52. Federer has won 23, Djokovic just 12.
The playing conditions then were also generally much quicker.
“Indoor tennis was totally different,” said Brad Gilbert, a coach and ESPN analyst whose playing career lasted from 1982 to 1995. “I do think the slower indoor courts today give everybody more of a chance.”
Federer, with his precise serve and knack for blocking huge serves back into play, presumably would have been a health hazard in that era, too.
Lendl was 341-70: a winning percentage of 83, which ranks second in the Open era indoors. McEnroe was 419-72: a winning percentage of 85.3, which ranks first by a significant margin, and he won five WCT Finals, which were the prestigious culmination of the WCT circuit, a rival of the traditional tour.
The only other men with an Open era winning percentage over 80 percent indoors in singles are Connors at 82 percent (469-103), Federer and Bjorn Borg at 80.6 percent (216-52).
Boris Becker, a tremendous indoor player, is next and close at 79.8 percent (297-75). And then there is Pete Sampras, whose five-set victory over Becker in Hanover, Germany, in the 1996 ATP final is the best indoor match this correspondent has seen in person. Sampras was 213-61 (77.7 percent) and won five year-end championships.
As in the GOAT debate, it is easier to compare and contrast indoor play in the Open era, when the major events were open to all instead of only to amateurs. But champions like Jack Kramer who turned professional before 1968 had ample opportunity to play indoors. As they barnstormed, they often played in a new city each night, traveling with a canvas court that could be rolled out and then rolled up and moved to the next venue.
“They could put it down on wood or cement, they even put it on ice a few times,” Rod Laver said in an interview. “In Madison Square Garden, when hockey season was on, they’d roll it out the day before, and your ankles were pretty much frozen by the time you came off court.”