While Nadal improved to 7-0 lifetime versus Gulbis, it was a typically close match between the two. This was only the second time in the series that Gulbis fell in straight sets and it took Nadal nearly 2 hours to finish off the win.
Nadal recorded a pair of breaks in each set but only won six more points over the course of the match than Gulbis, who was seeded seventh in Doha.
The win leaves Nadal as the only seeded player heading into the semifinals.
He will next take on Peter Gojowczyk, who defeated Dustin Brown 6-3, 3-6, 7-6 (7-5) in a battle of German qualifiers. While Gojowczyk's match went to a third-set tiebreaker, it was only 3 minutes longer than Nadal's.
Gael Monfils capped quarterfinal play with an efficient 6-2, 6-1 romp over Daniel Brands. Monfils lost 10 points in eight games serving and breezed through the win in 49 minutes.
Monfils is 4-0 lifetime versus Mayer, including a straight-set win last year in Stuttgart, Germany. Nadal has never before faced the No. 162nd-ranked Gojowczyk