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|Kei Nishikori eased into the Rotterdam Open quarter-finals. (AFP/Robin van Lonkhuijsen)|
Asia's top player, the world number seven, needed an hour and a quarter to advance as he booked a third victory against former French Open semi-finalist Gulbis, who was a lucky loser at the Ahoy Arena.
Nishikori will on Friday meet Hungarian Marton Fucsovics, who defeated Nikoloz Basilashvili of Georgia 7-6 (7/1), 6-1, for a place in the semi-finals.
Nishikori, who won his 12th ATP title in Brisbane earlier this year, earned his 10th victory of the season against a single loss - when he retired in the Australian Open last eight against Novak Djokovic.
"This was so much better than my first match," Nishikori said. "I was returning well and putting him under pressure.
"The second set was tight for a while but I was not worried even after he got the break back.
"I was able to play good tennis today, I'm very happy with my level."
Earlier, Jo-Wilfried Tsonga won a lengthy opening set and then accelerated past Dutchman Tallon Griekspoor for a quarter-final place.
The Frenchman's 7-6 (7/5), 6-1 victory was a match of two parts, with the labourious first set taking 53 minutes while the second lasted just 23.
Tsonga, the 2017 champion in Rotterdam, will keep up his bid for a second straight trophy after winning Montpellier at the weekend as he continues his comeback from a seven-month absence last season due to knee surgery and recovery.
The 33-year-old also looks set for another ranking rise from world number 140 as his comeback progress intensifies.
"At the start I played well, but he was solid also," Tsonga said after seeing off the 211th-ranked Dutchman.
"I took care of my serve and stuck to the game plan. I had my opportunities and he may have missed one or two balls more in the tiebreaker. I took my chances there."
Tsonga next faces fifth-seeded Russian Daniil Medvedev who defeated Spanish veteran Fernando Verdasco for the second time in three meetings, posting a 6-2, 7-5 win.
Medvedev, a Rotterdam quarter-finalist on his 2018 debut, has carried the momentum he gained from his fourth career title last weekend in Sofia.
The 23-year-old improved to 13-2 on the season.
Bosnian Damir Dzumhur followed up on his second-round upset of third seed Stefanos Tsitsipas, advancing past Kazakh Mikhail Kukushkin 6-4, 7-6 (7/3).
Dzumhur will be playing his first quarter-final of the season when he takes on French crowd-pleaser Gael Monfils.
Second round results:
Kei Nishikori (JPN) bt Ernests Gulbis (LAT) 6-1, 6-4
Marton Fucsovics (HUN) bt Nikoloz Basilashvili (GEO) 7-6 (7/1), 6-1
Damir Dzumhur (BIH) bt Mikhail Kukushkin (KAZ) 6-4, 7-6 (7/3)
Daniil Medvedev (RUS) bt Fernando Verdasco (ESP) 6-2, 7-5
Jo-Wilfried Tsonga (FRA) bt Tallon Griekspoor (NED) 7-6 (7/5), 6-1