The world number three opened up a two-shot lead at the majestic Royale Jakarta Golf Club over highly-rated Arnond Vongvanij of Thailand.
A rejuvenated Westwood, 38, who took a week's break after his third place finish at the US Masters, birdied the opening three holes and added three birdies and one bogey on his homeward nine.
"I had a break last week and hardly hit any balls," he was quoted as saying in a press release from the Asian Tour.
"I played a lot of golf earlier this year and needed a rest. It was good to go home and recharge my batteries because the Masters always takes a lot out of you. It is physically and mentally very demanding."
Westwood, who held the world number one spot after victory at the Indonesia tournament last year, was caught off guard at the 14th hole where he dropped his second bogey on what was a wind-swept day.
"This is quite a tricky test because the greens are very undulating. You got to have your A-Game with you," he said.
"(The wind direction) changed on 14 and it caught me by surprise and ended up with a bogey where I three putted."
Asian Tour rookie Arnond, who has two top-10 finishes this year, credited his straight shooting game for his strong card.
"I'm happy but not surprised. I feel that my game in the last couple of weeks have been really close to where I want it. I hit my driver fairly straight and didn't get into too much trouble," said the 23-year-old.
Veteran Boonchu Ruangkit of Thailand, former Asian Tour number one Jyoti Randhawa and Shiv Kapur of India, Lee Sung of Korea, Zaw Moe of Myanmar and Sam Cyr of the United States were bunched up in third place on 68.
Scores after round one of the CIMB Niaga Indonesian Masters at the par 72, 7,262-yard Royale Jakarta course:
65 - Lee Westwood (ENG)
67 - Arnond Vongvanij (THA)
68 - Lee Sung (KOR), Boonchu Ruangkit (THA), Zaw Moe (MYN), Jyoti Randhawa (IND), Sam Cyr (USA), Shiv Kapur (IND)
69 - Park Hyun-Bin (KOR), Berry Henson (USA), Thitiphun Chuayprakong (THA), Masanori Kobayashi (JPN), Angelo Que (PHI), Rick Kulacz (AUS)