Why is "lose" used instead of "lost"?

It's in the present tense - he watched as it happened. That goes for both - lose and land... it was happening ergo present tense.

Just to clarify, isn't the sentence in the past tense.

Thanks, hightor. Isn't the sentence in the past tense? Sorry for being unable to understand.

A group of around 10 people standing on a circular platform below rushed to the edge as they witnessed the lad lose his grip and land amid a sea of cricket fans.

But it wouldn't make any sense if you extended the past tense to the event as it happened:

A group of around 10 people standing on a circular platform below rushed to the edge as they witnessed the lad lost his grip and landed amid a sea of cricket fans.

In order to use the past tense you would need something like this:

A group of around 10 people standing on a circular platform below rushed to the edge as the lad lost his grip and landed amid a sea of cricket fans.

But once you add the fact that they were witnessing this event it moves into the historical present tense.

Did you click on the highlighted link?

Thanks, hightor.

I read your reply again. I got it.