AnOctorokForDinner
Talk Tennis Guru
Fedr - 194, between 2004 RG 3R (lost to Kuerten 6-4 6-4 6-4) and 2006 Cincy 2R (lost to Murray 7-5 6-4) - more than two full seasons
Borg - 165*, between 1978 WCT Challenge Cup SF (lost to Fleming 6-5 6-5) and 1980 Masters RR (tanked to Mayer 6-0 6-3) - almost two full seasons
*excludes two walkovers on his part and a retirement up 4-1 in the first set
JMac - 124, between 1983 Cincy F (lost to Wilander 6-4 6-4) and 1984 Davis Cup F (lost to Sundström 13-11 6-4 6-3) - 1 1/3 full season
Lendl - 102, between 1988 Stratton Mountain 2R (lost to Roger Smith 6-2 6-3) and 1989 Masters SF (lost to Edberg 7-6(5) 7-5) - 1 1/3 full season
Lendl (2) - 101, between 1981 Davis Cup QF (lost to Connors 7-5 6-4) and 1982 MC F (lost to Vilas 6-1 7-6 6-3)
Couldn't find any other such streak lasting more than a 100 matches (by a male player).
Other notable players' bests for reference:
Connors - 73**, between 1974 Toronto 3R (lost to Juan Gisbert 7-6 6-2) and 1975 Nottingham QF (lost to Tanner 7-5 6-3)
**excludes four walkovers (lolz)
alternatively, 66* between 1973 Masters SF (lost to Nastase 6-3 7-5) and 1974 Toronto 3R (lost to Gisbert 7-6 6-2)
*excludes one walkover
The Nadal - 71, between 2005 Valencia QF (lost to Andreev 7-5 6-2) and 2006 IW SF (lost to Blake 7-5 6-3). Ombillible but true, "babydal" gets peak resilience.
Agassi - 65, between 1994 Atlanta QF (lost to Washington 6-4 7-5) and 1995 IW F (lost to Sampras 7-5 6-3 7-5)
Djokovic - 59, between 2010 WTF SF (lost to Federer 6-1 6-4) and 2011 Cincy F (lost to Murray 6-4 3-0 ret.); equally, between 2013 Wimbledon F (lost to Murray 6-4 7-5 6-4) and 2014 MC SF (lost to Federer 7-5 6-2)
Murray - 49, between 2016 AO F (lost to Djokovic 6-1 7-5 7-6(3)) and 2016 Cincy F (lost to Cilic 6-4 7-5)
Vilas - 46, same as his peak win streak; between 1977 Wimbledon 3R (lost to Billy Martin 6-2 6-4 6-2) and 1977 Aix-en-Provence F (lost to spaghetti!Nastase 6-1 7-5 ret.)
Sampras - 42, between 1995 Rome 1R (lost to Santoro 6-4 6-3) and 1995 Essen SF (lost to Muster 7-6(6) 6-2)
Becker - 36, between 1989 Hamburg SF (lost to Horst Skoff 7-6 6-2) and 1989 Stockholm 3R (lost to Jan Gunnarsson 6-3 7-5)
Muster - 35, same as his peak win streak; between 1995 Davis Cup QF (lost to Edberg 6-4 6-2) and 1995 Gstaad 1R (lost to Corretja 7-5 6-1)
Borg - 165*, between 1978 WCT Challenge Cup SF (lost to Fleming 6-5 6-5) and 1980 Masters RR (tanked to Mayer 6-0 6-3) - almost two full seasons
*excludes two walkovers on his part and a retirement up 4-1 in the first set
JMac - 124, between 1983 Cincy F (lost to Wilander 6-4 6-4) and 1984 Davis Cup F (lost to Sundström 13-11 6-4 6-3) - 1 1/3 full season
Lendl - 102, between 1988 Stratton Mountain 2R (lost to Roger Smith 6-2 6-3) and 1989 Masters SF (lost to Edberg 7-6(5) 7-5) - 1 1/3 full season
Lendl (2) - 101, between 1981 Davis Cup QF (lost to Connors 7-5 6-4) and 1982 MC F (lost to Vilas 6-1 7-6 6-3)
Couldn't find any other such streak lasting more than a 100 matches (by a male player).
Other notable players' bests for reference:
Connors - 73**, between 1974 Toronto 3R (lost to Juan Gisbert 7-6 6-2) and 1975 Nottingham QF (lost to Tanner 7-5 6-3)
**excludes four walkovers (lolz)
alternatively, 66* between 1973 Masters SF (lost to Nastase 6-3 7-5) and 1974 Toronto 3R (lost to Gisbert 7-6 6-2)
*excludes one walkover
The Nadal - 71, between 2005 Valencia QF (lost to Andreev 7-5 6-2) and 2006 IW SF (lost to Blake 7-5 6-3). Ombillible but true, "babydal" gets peak resilience.
Agassi - 65, between 1994 Atlanta QF (lost to Washington 6-4 7-5) and 1995 IW F (lost to Sampras 7-5 6-3 7-5)
Djokovic - 59, between 2010 WTF SF (lost to Federer 6-1 6-4) and 2011 Cincy F (lost to Murray 6-4 3-0 ret.); equally, between 2013 Wimbledon F (lost to Murray 6-4 7-5 6-4) and 2014 MC SF (lost to Federer 7-5 6-2)
Murray - 49, between 2016 AO F (lost to Djokovic 6-1 7-5 7-6(3)) and 2016 Cincy F (lost to Cilic 6-4 7-5)
Vilas - 46, same as his peak win streak; between 1977 Wimbledon 3R (lost to Billy Martin 6-2 6-4 6-2) and 1977 Aix-en-Provence F (lost to spaghetti!Nastase 6-1 7-5 ret.)
Sampras - 42, between 1995 Rome 1R (lost to Santoro 6-4 6-3) and 1995 Essen SF (lost to Muster 7-6(6) 6-2)
Becker - 36, between 1989 Hamburg SF (lost to Horst Skoff 7-6 6-2) and 1989 Stockholm 3R (lost to Jan Gunnarsson 6-3 7-5)
Muster - 35, same as his peak win streak; between 1995 Davis Cup QF (lost to Edberg 6-4 6-2) and 1995 Gstaad 1R (lost to Corretja 7-5 6-1)
