While Sindhu will fancy her chances, retaining the title will still be much easier said than done as she still has to face the likes of ninth seeded Thai Pornpawee Chochuwong, top seed Tai Tzu Ying of Chinese Taipei and Korea's teen sensation An Seyoung.
World number 14 Srikanth, who had reached the knockout stage in the 2014 edition of the year-ending event, went down 18-21 7-21 to three-time junior world champion Vitidsarn in a 46-minute men's singles match.
BWF World Championships: PV Sindhu gets first round bye in tournament draw, likely to face Tai Tzu Ying in quarters
Sindhu, who won a bronze in the Tokyo Olympics, has been seeded sixth in the 12-19 December showpiece event.
BWF World Tour Finals: Sindhu, Srikanth make winning start; Ponnappa-Sikki Reddy lose in women's doubles
Former world No 1 Kidambi Srikanth beat France's Toma Junior Popov 21-14 21-16 in 42 minutes in his opening match of group B.
Lakshya, who had finished runners-up at Dutch Open last Sunday, dished out a superb performance to outclass national champion Sourabh 21-9, 21-7 in 26 minutes.
Saina Nehwal had to concede her match after losing the first game 20-22 against Spain's Clara Azurmendi as she developed discomfort in her groin area.
It was India's second consecutive defeat after losing the Sudirman Cup Group A opener to Thailand.
India have been drawn in Group A alongside defending champions China, three-time semi-finalists Thailand and hosts Finland in the BWF tournament, which comprises two singles and three doubles matches in a tie.
The prestigious World Mixed Team Championship is scheduled to be held in Vantaa, Finland from 26 September to 3 October.
BAI to conduct selection trial on 28 August to pick teams for Sudirman Cup, Thomas and Uber Cup Finals
Two-time Olympic medallist PV Sindhu, Saina Newhal, Tokyo Olympian B Sai Praneeth, Kidambi Srikanth and world No. 10 Chirag Shetty and Satwiksairaj Rankireddy have been selected directly for the two events.
The Indian pair lost in the final to top seeds Amalie Magelund and Freja Ravn of Denmark.
Orleans Masters: Saina Nehwal bows out in semis; Krishna Prasad Garaga and Vishnu Vardhan Goud Panjala enter maiden final
Krishna and Vishnu cheered up the Indian camp with a 21-17 21-17 win over the English pair of Callum Hemming and Steven Stallwood in a 35-minute semi-final clash to reach their maiden final of a Super 100 event
Orleans Masters: Saina Nehwal wins thriller against Iris Wang to move to semi-finals as Kidambi Srikanth crashes out
The fourth-seeded Indian, who had withdrawn from the All England Championships last week due to an injury, had to dug deep to script a 21-19 17-21 21-19 win over world No 36 Wang
Fourth seeded Saina, who is desperately seeking ranking points to make it to her fourth Olympics, took just 21 minutes to beat Rachael 21-9 21-5 and set up a clash with France's Marie Batomene.
All England Open 2021: Mark Caljouw beats Lakshya Sen in quarter-finals; Ashwini Ponnappa-N Sikki Reddy bow out
Sen's defeat to Caljouw, with the match lasting a total of 55 minutes, saw the end of Indian challenge in men's singles.
All England Open 2021: Ashwini Ponnappa-N Sikki Reddy pair enters quarters; Sameer Verma, Satwiksairaj Rankireddy-Chirag Shetty eliminated
The world number 30 pair of Ashwini and Sikki took just 33 minutes to see off the 13th ranked duo of Gabriela and Stefani 21-17 21-10 in the second-round match on Thursday night.
All England Open 2021: PV Sindhu cruises past Soniia Cheah; Kidambi Srikanth, Parupalli Kashyap suffer early exits
The Indian women's duo of Ashwini Ponnappa and N Sikki Reddy and men's doubles pair of Satwiksairaj Rankireddy and Chirag Shetty also progressed to the second round with straight game wins.
Swiss Open: PV Sindhu, Kidambi Srikanth through to round of 16; Satwiksairaj Rankireddy-Chirag Shetty win
While world champion Sindhu beat Yigit Neslihan of Turkey 21-16, 21-19 in her first-round match that lasted 42 minutes, Srikanth staved off a spirited challenge from compatriot Sameer Verma to make a winning start to his campaign.
Saina Nehwal and Co leave for Thailand to compete in two Super 1000 events; PV Sindhu to fly from London
The Indian badminton team, including Olympic hopefuls Saina Nehwal, B Sai Praneeth and Kidambi Srikanth, on Sunday left for Thailand to compete in back-to-back BWF World Tour Super 1000 events.
N Sikki Reddy relieved after testing negative for COVID-19, says abuse directed at her on social media left her ‘shocked’
Sikki said it was a difficult 48 hours for her and family as she also had to deal with verbal abuse from fans on social media but she managed to keep her cool.