Andrea Bocelli Sings 'Happy Birthday' to TMZ Celebrity Tour Bus Rider

TMZ Celebrity Tour bus riders were in a huge treat to end the week -- 'cause Andrea Bocelli was in town ... and he came aboard to put his pipes on display for one lucky rider!

We ran into the legendary Italian tenor out in the streets of Hollywood Friday -- where we happened to be doing our TMZ Celebrity Tour ... and one of our guides spotted Andrea and his son, Matteo, in the heart of the city, opting to hop off and interview him.

It made for a pretty epic conversation if we do say so ourselves. Check it out ... not only did AB give us the time of day -- telling us all about his upcoming projects -- but he also obliged us by coming on the bus and chopping it up with the riders.

The dude was super sweet ... and once he was on the bus -- he did someone a huge favor.

He didn't do "Ave Maria" here -- but Andrea did sing the next best thing ... and that would be "Happy Birthday" to one of our tour guide guests. Of course, phones were out for this one.

Just goes to show ... you really do see famous people on the TMZ Celebrity Tour -- and it's 100% worth its weight in gold, especially when awesome stuff like this happens. We'll admit ... even we were a little star-struck with Andrea's pop-in here, 'cause the dude's an absolute icon.