Rafael Nadal secured his first title of the year and 87th of his career, denying Tsitsipas back-to-back triumphs following his maiden Masters 1000 title at Monte Carlo last week.
Nadal, who captured his first Barcelona title as an 18-year-old at the 2005 tournament, will face Pablo Carreno Busta in Saturday's semi-finals.
The world number three triumphed 6-0, 2-6, 6-2 over 2014 and 2015 champion Nishikori and next faces Britain's Cameron Norrie, a fellow left-hander, for a place in the semi-finals.
The 34-year-old Nadal, who suffered a surprise quarter-final exit at the hands of Andrey Rublev in the Monte Carlo Masters quarter-finals last week, dropped the first set against the world 111 before pulling through 3-6, 6-2, 6-4.
Briton Dan Evans says a sports psychologist described him as one of the angriest people he had ever met during his one-year ban from tennis for cocaine use in 2017.
Coronavirus pandemic: ATP suspends men’s tennis tour for six weeks; Monte-Carlo Masters, Barcelona Open impacted
The ATP suspended its professional men’s tennis tour for six weeks on Thursday, citing concerns over the coronavirus outbreak. Impacted events include the Monte-Carlo Masters, the Barcelona Open and the Hungarian Open.
David Ferrer managed to make a name for himself in era of superstars and that is his biggest victory
In a professional career spanning almost twenty years, Spaniard David Ferrer managed to maintain a high-level of tennis throughout because of his dogged determination.
Barcelona Open: Dominic Thiem thumps Daniil Medvedev to clinch title and bolster Roland Garros credentials
Dominic Thiem overpowered Daniil Medvedev to win the Barcelona Open title, boosting his credentials as a potential Roland Garros champion.
Barcelona Open: Dominic Thiem's win over Rafael Nadal proves he's ready to take over rival's clay-court king mantle
Dominic Thiem's fitness, gameplay and mind are all aligned perfectly to be able to shape his future and come close to sharing the rarefied air of Rafael Nadal.
While Medvedev is on this journey of improvement, maybe we should learn to be less shocked whenever he shows signs of emotion on the court. He is continuously evolving after all, and isn’t evolution the one thing that separates humans from machines?
Barcelona Open: Rafael Nadal's worries pile up as Dominic Thiem stuns 11-time champion; Daniil Medvedev also in final
"I'm happy with how I played, I was competitive for the first time (in a few matches). Things were positive even if I lost," Rafael Nadal insisted despite the defeat.
Barcelona Open: Rafael Nadal inches closer to ATP record after seeing off Jan-Lennard Struff in quarter-finals
Rafael Nadal held off Jan-Lennard Struff 7-5, 7-5 in the Barcelona Open quarter-finals on Friday to move closer to setting an ATP record as the first player to win the same event 12 times.
Barcelona Open: Rafael Nadal pays emotional tribute to compatriot David Ferrer after reaching quarter-finals
Rafael Nadal showed no mercy towards an emotional David Ferrer as he eased into the quarter-finals of the Barcelona Open on Thursday, describing his compatriot who is winding down his career as a "pleasure to watch".
Rafael Nadal survived an early scare at the Barcelona Open on Wednesday by coming from a set down to beat Argentina's Leonardo Mayer and seal his place in the last 16
Barcelona Open: Alexander Zverev stunned by lucky loser Nicolas Jarry in second round; Dominic Thiem advances
Alexander Zverev crashed out of the Barcelona Open in the second round on Tuesday, continuing his troubled start to 2019.
Barcelona Open: Rafael Nadal says he won't dwell on Monte Carlo semi-final loss, looks to bounce back in Catalan capital
Rafael Nadal said he won't waste time dwelling on his Monte Carlo semi-final loss but instead will concentrate on bouncing back at the Barcelona Open.
Novak Djokovic, whose latest comeback in Barcelona last week ended after one match, believes he will improve before Roland Garros.
It was World No 1 Nadal's 55th career clay-court title while he also extended his winning streak on the surface to 46 consecutive matches.
Barcelona Open: Rafael Nadal notches 400th win on clay, sets up title clash with Greek teenager Stefanos Tsitsipas
World number one Nadal also extended his winning streak on clay to 44 consecutive sets as he downed Goffin for a third time in three meetings on the red dirt surface he has dominated for more than a decade.
Barcelona Open: Rafael Nadal beats Martin Klizan in quarters for his 399th win on clay; Dimitrov, Thiem lose
World number one Nadal was made to work by Slovakian qualifier Klizan, ranked at 140 and who had knocked out Novak Djokovic in the second round.