Zverev says he spends up to four hours each day working on weights and fitness.
Zverev, who had come through three five-set matches to reach the last eight, required a medical timeout at the start of the second set and was clearly affected in his first grand slam quarter-final.
The Austrian seventh seed, the last man to beat claycourt master Rafa Nadal on his favorite surface, displayed an impressive arsenal of weapons in the first two sets before regaining his composure after the loss of the third. As the score indicates, that was far less compelling, in part because Zverev felt pain in his left hamstring 10 minutes into the match and wound up having his leg taped by a trainer.
For the third consecutive spherical, Zverev rallied from a 2-1 deficit in units and this time it got here in opposition to a pal from his junior days, Karen Khachanov.
"It's going to be a very tough match against him". Thiem called their upcoming quarterfinal "the matchup most of the fans in Germany and Austria were hoping for when they saw the (French Open) draw".
In the third set, however, Nishikori stayed in touch as he served better and finished off points at the net, which unsettled Thiem who grew frustrated.
He followed up a wickedly spinning sublime backhand drop with another rasping one-handed backhand victor after saving a break point to lead 5-1 and sealing a crushing win with a straightforward backhand into an open court.
On Tuesday, he became the first Italian to reach the last four at a major in 40 years by beating 12-times Grand Slam champion and 2016 Roland Garros victor Novak Djokovic on Court Suzanne Lenglen.
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"I'm expecting another five-set match". It's the best, a five-set match.
Over on Lenglen, meanwhile, Madison Keys reached her first French Open semifinal by defeating unseeded Yulia Putintseva 7-6 (5), 6-4.
He will next face 2016 champion Novak Djokovic for a place in the final four.
Men's fourth round: No. 1 Rafael Nadal beat Maximilian Marterer 6-3, 6-2, 7-6 (4); No. 3 Marin Cilic beat No. 18 Fabio Fognini 6-4, 6-1, 3-6, 6-7 (4), 6-3; No. 5 Juan Martin del Potro beat No. 9 John Isner 6-4, 6-4, 6-4; No. 11 Diego Schwartzman beat No. 6 Kevin Anderson 1-6, 2-6, 7-5, 7-6 (0), 6-2.
"First week, my goal is to get to the second week", said 23-year-old Keys. Both have provided Nadal with challenges during this clay court season, although only Thiem has beaten him.
Thiem will be looking for revenge as he was handed a straight-game thrashing the last time the two youngsters met - Madrid Open final in May.
"If Thiem comes and makes the first hour, hour and a half really physical and does not miss too much, then I actually think in this situation, because of the circumstances, this is a good chance for him to beat Zverev", the Swede said. In all three matches, the lanky player was trailing two sets to one before making strong comebacks.