Despite struggling on his second serve and being broken in both sets, the Spanish world number one battled to a 7-5 6-4 victory – sealing the win with a delicate low volley at the net.
"This is a tournament I really like to play and I've won here before and reached the final so I'm really happy," he said.
"I'm just thinking about Doha at the moment and not looking ahead to the Australian Open."
Gael Monfils brushed aside Daniel Brands 6-1 6-2 to book a last four meeting with Florian Mayer, who backed up his surprise win over third seed Andy Murray with a 6-4 6-7 6-4 victory over Victor Hanescu.
Results from the Qatar Open Men's Singles quarter-finals matches
Gael Monfils (France) beat Daniel Brands (Germany) 6-2 6-1
Florian Mayer (Germany) beat Victor Hanescu (Romania) 6-4 6-7(8) 6-4
1-Rafa Nadal (Spain) beat 7-Ernests Gulbis (Latvia) 7-5 6-4
Peter Gojowczyk (Germany) beat Dustin Brown (Germany) 6-3 3-6 7-6(5
- Sports & Recreation
- Peter Gojowczyk
- Gael Monfils
- Florian Mayer
- Daniel Brands