DynamoDB Upsert with condition on numeric attribute

Is it possible to do an upsert (insert if the item does not exist, update otherwise) with a condition on a specific attribute using the Update() call?

For example I have a UpdatedTime that I only want to allow to be greater when upserting, otherwise do nothing. This is fine if the item already exists, but the conditional will fail if there is no existing item. Is it possible to do this with a single call to DynamoDB? The alternative is to check the error and then do an Insert(), but now I have a race to deal with.