Celtic prepared for their Champions League trip to Kazakhstan next week by losing a friendly 6-5 on penalties to Leicester after a 1-1 draw.
Brendan Rodgers' side - who play FC Astana on Wednesday - dominated possession for large periods against the Premier League champions.
Riyad Mahrez put Leicester ahead with a superb goal only for Eoghan O'Connell to curl in from 25 yards.
Kasper Schemichel then saved a James Forrest penalty in the shootout.
Celtic still have potentially four qualifiers to play before reaching the group stage of the Champions League, beginning with a 3,000-mile trip to Astana, in Kazakhstan, this week. But Rodgers played a strong side for this International Champions Cup friendly.
Mahrez won the Professional Footballers' Association Player of the Year award last season and the Algerian showed his qualities again by cutting inside from the right wing and curling superbly into the top corner in the 46th minute.
Leicester - who named three new signings in their starting XI - will be desperate to keep Mahrez at the King Power Stadium. He has been strongly linked with a move to Arsenal, but manager Claudio Ranieri says "it will be better for him to stay with me".
'Did you see how Riyad is happy?'
Mahrez, 25, was signed by the Foxes from Le Havre for £400,000 in 2014 but reports in France on Saturday suggested he was set to join Arsenal for up to £50m.
He was the outstanding player on display in Glasgow, and after the match Ranieri again repeated that the winger would be staying.
"Did you see how Riyad is happy?" said Ranieri.
"He played so well, worked so hard for the team, he scored a goal and he stays with us.
"It is very important (to keep him); he is one of our stars.
"Other teams maybe have more stars and maybe he doesn't play at the same level, because maybe is on the bench so it is much better he stays with me."
'A winning mentality'
Celtic boss Brendan Rodgers saw 20-year-old centre-back O'Connell guide in off a post in the 59th minute after Scott Brown's shot had been blocked to level the game, which went straight to penalties after 90 minutes.
The first 10 spot kicks were converted before Forrest saw his effort saved and Daniel Amartey scored to give Leicester a bonus point in the competition.
Celtic have their important Champions League tie in between a friendly game with Barcelona in Dublin next weekend, while Leicester face Paris St-Germain in California next Saturday.
Rodgers was impressed with his side's display and is also confident of finalising the signing of former Liverpool defender Kolo Toure this weekend.
"It was an excellent game," he said. "For a pre-season game, it was at a good level and good intensity. Leicester looked fit and strong and ready for another challenge.
"We were excellent. Mahrez scored a fantastic goal but our reaction was really good. The mentality we have tried to create is a winning one. Even in pre-season, that's important.
"We came from a goal behind and that is important."
Rodgers also indicated that a deal is close to sign former Arsenal and Manchester City defender Kolo Toure, released by Liverpool at the end of last season.
"Hopefully Kolo Toure will be signed by Sunday and he will be great for us," Rodgers said. "I bought him to Liverpool and he did great. He looks after himself and is in good condition. He will be a valuable player for us."
Celtic: Gordon (Fasan 62); Janko, Lustig (Ambrose 46), O'Connell; Roberts (Forrest 62), McGregor (Johansen 62), Brown (Allan 75), Armstrong (Rogic 75), Izaguirre; Griffiths (Christie 75), Dembele (Ciftci 62).
Leicester: Zieler (Schmeichel 46); Amartey, Hernandez (Okazaki 62), Morgan (Wasilewski 62), Chilwell; Mahrez (Fuchs 70), Mendy (Simpson 46), Drinkwater, Albrighton (Schlupp 46); Ulloa (Huth 62), Gray (Musa 46)