Thai League (T1) leaders Buriram United were dumped out of their second cup competition in a week as a late Felipe Azevedo strike gave Chiang Rai United a 1-0 victory in the Thai FA Cup quarterfinals.
The visitors missed several chances in a scrappy game before Azevedo struck with nine minutes remaining.
Having lost to Muang Thong United in the League Cup last week, Buriram will look to wrap up the T1 title as soon as possible, requiring two wins from the final four games.
Buriram made a lively start and home keeper Chatchai Budprom had to deal with Go Seul-Ki's strike from edge of the box after three minutes.
But the opening half was more notable for the numerous fouls than scoring opportunities.
Jaja Coelho twice almost opened the scoring for Buriram early in the second half. First, he headed Supachok Sarachat's cross off the bar, and he came close again when he shot just wide from 14 yards out.
Jaja then looked certain to grab the first goal with 14 minutes remaining when Chatchai parried Bastos' header into his path. But the striker couldn't get his body over the ball and fired over.
Chiang Rai then stunned their opponents by taking the lead in the 81st minute. After a mix-up in the Buriram defence, the ball was fed out to Chaiyawat Buran. The substitute crossed low from the left and Azevedo stole in to fire home at the near post.
Chiang Rai advance alongside Muang Thong United, who needed a Teerasil Dangda goal to see off PTT Rayong.
Bangkok United won 1-0 after extra time at Suphanburi and fourth-tier side JL Chiang Mai United make up the last four, having beaten Nongbua Pitchaya 2-1.
Meanwhile, Buriram will aim to get back to winning ways and take a step closer to the T1 title when they visit Suphanburi on Sunday.