Taylor Fritz put Team World to within one win of capturing their first Laver Cup after the American saw off Europe's Dominic Thiem 7-5 6-7 (3/7) 10-8 in a barnstorming match in Geneva that leaves his side 11-7 up with two matches to play.
World had come into the day 7-5 down but were boosted by Rafael Nadal's withdrawal with a hand injury.
Fritz, himself a replacement for injured Nick Kyrgios, followed up on John Isner and Jack Sock's 5-7 6-4 10-8 defeat of Roger Federer and Stefanos Tsitsipas with a surprise tie-break triumph over world number five Thiem.
"That was such a big match for Team World to win, we really needed that," said Fritz.
"Now we're four points in the lead, one match from the win. It was so crucial.
"This has to be one of the biggest wins of my career. It means so much more when you're playing for other people as well."