Former world number ones Roger Federer and Novak Djokovic lost in the Laver Cup doubles but Team Europe lead Team World 3-1 after day one of the event.
The pair were beaten 6-7 6-3 10-6 by American Jack Sock and South Africa's Kevin Anderson in Chicago.
Federer and Djokovic took a while to settle into the format and won the opening set but their opponents soon found their rhythm.
Earlier in the singles, Britain's Kyle Edmund defeated Sock 6-4 5-7 10-6.
There were also wins for Bulgarian Grigor Dimitrov who beat America's Frances Tiafoe 6-1 6-4 and for Belgium's David Goffin, who defeated Diego Schwartzman of Argentina 6-4 4-6 11-9.
But it was the doubles match which created the most excitement.
Djokovic accidently hit his partner with the ball in the third game but both players laughed off the incident, much to the amusement of the crowd.
They claimed the first set on a tie-break but Sock's experience as a three-time doubles major champion shone through at key moments as he impressed at the net.
Sock and Anderson levelled the match and having seen their team-mates lose in two deciders earlier in the day, the pair made sure not to waste their opportunity when Federer double-faulted at 4-4.
An impressive return winner from Sock sealed the set 10-6, giving captain John McEnroe some relief before day two where two points will be up for grabs.