The Tasmanian, who sat out the team presentation and pre-Vuelta press conference due to a bout of gastro, stopped the clock in the opening 8km ITT at 10 minutes and 30 seconds.
"I didn't feel great today but I knew that I wasn't coming into the race in my best form," he said. "I know I am going to take a bit of a kicking over the first two weeks. But I think I should be better in the last week.
"Mentally, it was hard to get over the crash in July and it wasn't easy to restart or find my momentum but I have still done some good training. I am definitely not as good as I was at the Tour but hopefully by the end, I can be in good shape. I would love to be able to take a stage win in the final week. It looks really hard on paper and that's my big motivation."
"I look forward to building on my form especially for the UCI World Championships over the next three weeks."
Porte trails his team mate Rohan Dennis by 51 seconds and is at the bottom end of a long list of GC favourites who could place in the top 10 overall come Madrid in three weeks.
But the Australian isn't too far behind the rivals who'll relish the tough, final week of climbing.
The top contenders after Stage 1 of the 2018 La Vuelta a España:
Rohan Dennis (BMC)
Michal Kwiatksowski (Sky) +6
Wilco Kelderman (Sunweb) +22
Alejandro Valverde (Movistar) +24
Bauke Mollema (Trek Segafredo) +27
Steven Kruijswijk (LottoNL Jumbo) +29
Simon Yates (Mitchelton-Scott) +29
Nairo Quintana (Movistar) +30
Ilnur Zakarin (Katusha) +30
Rafal Majka (Bora) +34
Miguel Angel Lopez (Astana) +35
Dan Martin (UAE) +38
Fabio Aru (UAE) +39
Thibaut Pinot (FDJ) +40
Vincenzo Nibali (Bahrain Merida) +40
George Bennett (LottoNL Jumbo) +42
Rigoberto Uran (EF Drapac) +45
David De La Cruz (Sky) +45
Jack Haig (Mitchelton-Scott) +47
Richard Carapaz (Movistar) +48
Richie Porte (BMC) +51