To root for the underdog

What is the definition of an underdog in hockey?

What is the reason to root for the underdog in a tennis match?

In sports like tennis, people root for the underdog that end up facing the player or team that always wins. Most people root for the underdog because it is refreshing to see a surprising win rather than the obvious victory from someone everyone expects to win.

Chornomorets – Obolon
Football. Ukrainian Premier League
Napoli – Calcio Como
Football. Serie A
Napoli – Como
Football. Italy. Serie A
Paderborn – Regensburg
Football. Germany. 2. Bundesliga
Karlsruher SC – Darmstadt
Football. Germany. 2. Bundesliga
Fenerbahce – Olympiakos
Basketball. Euroleague
Augsburg – B. Monchengladbach
Football. Germany. Bundesliga
Huesca – Cadiz CF
Football. Spain. LaLiga2
Verona – Venezia
Football. Serie A
Verona – Venezia
Football. Italy. Serie A
Marseille – Angers
Football. France. Ligue 1
Westerlo – Beerschot VA
Football. Belgium. Jupiler Pro League
Leganes – Valencia
Football. La Liga
Leganes – Valencia
Football. Spain. LaLiga
Sunderland – Leeds
Football. England. Championship
M. Evloev – A. Sterling
MMA. UFC 307
S. Thompson – J. Buckley
MMA. UFC 307
T. Means – C. McGee
MMA. UFC 307
R. Dolidze – K. Holland
MMA. UFC 307
N. Ball – R. Rios
Boxing. WBA Fight Night in Liverpool
I. Potieria – C. Almeida
MMA. UFC 307
A. Hubbard – A. Hernandez
MMA. UFC 307
K. Vieira – K. Harrison
MMA. UFC 307
J. Aldo – M. Bautista
MMA. UFC 307
R. Pennington – J. Pena
MMA. UFC 307
A. Beterbiev – D. Bivol
Boxing. Top Rank
P. Kincl – K. Engizek
MMA. Oktagon 62
F. Wardley – F. Clarke
Boxing. British and Commonwealth Titles
C. Eubank Jr. – K. Szeremeta
Boxing. IBO Title
J. Opetaia – J. Massey
Boxing. IBF Title
B. Royval – T. Taira
MMA. UFC Vegas 98
Borussia Dortmund – Celtic
Football. UEFA Champion's League