That's exactly what happened Saturday in London ... where none other than Tom Cruise himself dropped in for the ultimate round between tennis stars Ashleigh Barty and Karolína Plíšková ... with his 'M:I 7' leading lady costars, Hayley Atwell and Pom Klementieff, in tow.

It's rumored Hayley and Tom are actually dating right now -- and based on their body language before and during the match ... that may very well be true. They seemed chummy.

Anyway, the cameo made for a lot of fanfare ... people gave it up for Tom and co., offering up rounds of applause and stopping for photo-ops, etc. Of course, TC was graceful -- waving to the crowd ... and looking dapper as hell doing it. What else would you expect, right?

Interestingly ... the dude didn't seem too concerned with masking up, and neither did his cast members (or the Wimbledon audience at large). That's fine though -- the Wimbledon folks' mask policy is like a lot of other event venues lately ... ya must provide proof of vaccination, or a recent negative COVID test -- plus, wear masks while roaming the grounds.

So, it's all kosher. And, thankfully, it is ... otherwise the love-all spectators might've gotten a taste of what the 'Mission: Impossible' crew did last year while breaking protocol on set.