His Canadian opponent was forced to retire after suffering an injury to his upper right thigh in the first game of the second set.
The Spaniard, current champion, will face his compatriot Jaume Munar for a place in the final. Munar, number 64 in the world, qualified after eliminating Frenchman Kyrian Jacquet, 86th in the ranking.
The match was closely contested until the retirement of Shapovalov, who put Alcaraz in difficulties with the variety of his game, combining deep blows with drop shots, and power with spin.
Alcaraz, who saved four set points against Matteo Arnaldi on Saturday, saved another here at 5-4 down and then took the tiebreaker with a forehand winner before Shapovalov’s injury.
The Australian Open champion is playing his third tournament since recovering from a wrist injury that kept him off the court for several months, after reaching the quarterfinals of the United States Open last month.
In the other quarterfinal duels, Monegasque Valentin Vacherot (18th) beat Frenchman Arthur Fils (9th) 7-5, 4-6, 6-4, and will face Jiri Lehecka in the semifinals.
The Czech (23rd) eliminated the young Paraguayan Daniel Vallejo (56th) in less than an hour 6-1, 6-1.
— Tokyo tournament results.
– Men’s singles – Quarterfinals:
Carlos Alcaraz (ESP/N.1) defeated Denis Shapovalov (CAN) 7-6 (7/3), 2-1 and abandonment
Jaume Munar (ESP) vs. Kyrian Jacquet (FRA) 6-4, 6-2
Jirí Lehecka (CZE) and Daniel Vallejo (PAR) 6-1, 6-1
Valentin Vacherot (MON/N.8) to Arthur Fils (FRA) 7-5, 4-6, 6-4
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