"I came in playing garbage".
He added with a smile: "I mean, always, you know, when we play, it's always a battle".
"It's great for me to be in the semi-finals again, to defend that", he said.
'I think it will be an interesting match.
Having dispatched Karen Khachanov in straight sets 6-3, 6-4, 6-4, Murray used his courtside post-match interview to talk about the terror attacks at Manchester Arena and London Bridge that left 29 people dead and many more injured.
The Scot will now face the tournament's 2015 victor Wawrinka, who beat Marin Cilic 6-3, 6-3, 6-1, in what is a repeat of last year's semi-final showdown which Murray won in four sets before losing to Novak Djokovic in the final. A fired-up Murray reeled off the next five games to seal a commanding 6-1 set.
Nishikori didn't have it all his own way against Verdasco, failing to win a single game in a 26-minute first set, but he recovered his composure to oust the 33-year-old and advance.
Nadal, Thiem and Wawrinka are yet to drop a set this tournament while Murray has lost three, and their build-ups were rather different too.
The Austrian finished on his second match point.
Watch the best moments from the Andy Murray/Karen Kahchanov duel at the men's fourth round of Roland-Garros 2017 (Andy won 6:3, 6:4, 6:4).
Jelena Ostapenko was serving for the second set in her match against Caroline Woznacki after the Dane clinched the opener 6-4.
Nishikori got the better of Murray when the two met in the quarter-finals of last year's US Open and looked capable of repeating the feat early on as he consistently forced his opponent on to the back foot.
The Japanese twice came from behind in a third set that ebbed and flowed, only to surrender meekly during the subsequent breaker.
Women: 28- Carolina Garcia (France) bt Alize Cornet (France) 6-2, 6-4; 5-Elina Svitolina (Ukraine) bt Petra Martic (Croatia) 4-6, 6-3, 7-5; 3-Simona Halep (Rom) bt 21-Carla Suarez Navarro (Spain) 6-1, 6-1. "So I'm sure it's going to be a tough one, but I'll try to enjoy and try to win the match".