Players who won as much or more than matches lost at event.

[BGod]

Players who won as much or more than matches lost in Slam events:
Nadal at French Open: 9-1
Sampras at Wimbledon: 7-7
Borg at French Open: 6-2
Borg at Wimbledon: 5-4
Connors at Australian Open: 1-1

Near Misses:
Djokovic at Australian: 5-6
Agassi at Australian: 4-5


Now Slams are hard, so I looked at ATP Master Series events. A lot more such cases, but usually by the greatest of greats, and Nadal at the Clay events.

Nadal at Monte Carlo: 8-4
Nadal at Rome: 7-3
McEnroe at Wembley: 5-1
McEnroe at WCT Finals: 5-4
Djokovic at Miami: 5-5
McEnroe at Stockholm: 4-2
McEnroe at Philadelphia: 4-3
Djokovic at Tour Finals: 4-4
Borg at Boston: 3-0
Borg at Las Vegas: 2-0
Borg at Rome: 2-1
Borg at Tokyo: 2-2

Near Misses:
Federer at Tour Finals: 6-7
Sampras at Tour Finals: 5-6
Borg at Masters: 2-3

 

[spirit95]

Do you mean players who won events on 50% or more of the occasions they played?

 

[CHIP72]

Most of the players listed have the advantage of still playing in or not long after in their primes now (Nadal, Djokovic), retiring young (Borg), or playing infrequently at an event (Connors and to a lesser degree Agassi at the AO). The stats do show how dominant Nadal has been at many clay court events (evidenced by his way above ".500" record).