Coming into Roland Garros, Alcaraz had not played for a month because of pain in his right forearm and continued to wear a support throughout the tournament.
He missed the Italian Open as a consequence and arrived in Paris with only a run to the Madrid Open quarter-finals under his belt on clay.
In contrast, Zverev prepared by winning the prestigious title in Rome.
Their varying paths to the tournament – and the draw – meant it was a difficult final to call.
That remained the case when the pair started the deciding set.
By now Zverev had spent almost 24 hours on court in the tournament – having taken the longest route to the French Open final since records began in 1991.
A shocker of a game – including two poor volleys and a double faults – saw Zverev lose serve for 2-1, before Alcaraz had a missed second serve overruled on break point in the next game and went on to hold.
Olympic champion Zverev dipped into his mental and physical reserves again to create five break points, but could not take any of them as Alcaraz survived.
When Alcaraz broke for 5-2, with the help of a sensational backhand cross-court winner, Zverev looked utterly demoralised and could not recover.
“The fifth set, there was some unlucky moments,” said Zverev, whose defeat came two days after his trial over domestic abuse allegations was discontinued.
“I heard that at 2-1 the second serve was out.
“I break back there, then a fifth set can go the other way.”