By ESPNSTAR.com staff
Samuel Ajayi grabbed the only goal of the game at the LEO Stadium in the 33rd minute to give his side the most slender of leads ahead of next Wednesday's return encounter.
The result also means the Glass Rabbits remain on track in their bid to win a cup double, as they are currently also in the quarter-finals of the FA Cup, where they will meet Ayutthaya on October 3.
Muangthong United, who are currently ten points clear at the top of the Thai Premier League, fell to a stunning 2-1 defeat to TOT, courtesy of Takahiro Kawamura's injury-time winner from the spot.
TOT took the lead in the 58th minute through Thanakorn Dangthong, but the hosts looked to have done enough to force a draw when Dagno Siaka equalised two minutes later.
However, the visitors were awarded a penalty deep into injury-time, presenting Kawamura with the chance to grab the winner.
Defending Premier League champions Buriram United were held to a 1-1 draw by BEC Tero Sasana at the Thunder Castle Stadium.
Brazilian striker Cleiton Silva fired the Fire Dragons in front after five minutes when he found the back of the net, but Thai-American midfielder Anthony Ampaipitakwong scored in the 62nd minute to force a draw for the hosts.
Finally, Ratchaburi, who are the only non-Premier League side left in the competition, recorded a shock 2-0 win over Wuachon United.
Having been humbled by Buriram just last week as they fell to a 7-1 defeat in the fourth round of the FA Cup, Ratchaburi got off to a fine start when striker Douglas opened the scoring in the 11th minute.
Fellow Brazilian Wander Luiz then doubled their advantage three minutes after the half-hour mark to give the Dragons a comfortable lead ahead of next week's second leg at the Tinsulanon Stadium.
Wednesday's Thai League Cup Quarter-final 1st Leg Results
Ratchaburi 2-0 Wuachon United
Bangkok Glass 1-0 Chonburi
Buriram United 1-1 BEC Tero Sasana
Muangthong United 1-2 TOT