Alcaraz defeated on Laver Cup debut
The US pair's 7-6 (7/5), 6-4 win in the final match of the opening day in Berlin allowed Team World to level the tie overall at 2-2.
Fritz and Shelton fired 20 winners against world number two Zverev and third-ranked Alcaraz.
"We both served really well under pressure. Ben served incredibly well, so I didn't have to hit any volleys. That was huge," said Fritz playing for the first time since finishing runner-up to Jannik Sinner in the US Open final in New York 12 days ago.
Earlier Friday, Argentina's Francisco Cerundolo defeated Casper Ruud 6-4, 6-4 before Stefanos Tsitsipas pulled Team Europe level by seeing off Australia's Thanasi Kokkinakis 6-1, 6-4.
Grigor Dimitrov then edged Europe in front with a 7-6 (7/4), 7-6 (7/2) victory against Alejandro Tabilo.
There will be three more singles matches and one doubles match in Berlin on Saturday.
Team World is bidding to win a third consecutive Laver Cup this year after triumphing in London in 2022 and Vancouver last year.