what is the {start time} for {Novak Djokovic} has {occupation} as {tennis player} ?
When did Novak Djokovic become a professional tennis player?
SELECT ?value WHERE { wd:Q5812 p:P106 ?s . ?s ps:P106 wd:Q10833314 . ?s pq:P580 ?value}
select distinct ?value  where {
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> <http://wikidata.dbpedia.org/resource/Q5812> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#predicate> <http://www.wikidata.org/entity/P106> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> <http://wikidata.dbpedia.org/resource/Q10833314> .
?statement <http://www.wikidata.org/entity/P580> ?value. 
} 

Novak Djokovic became a pro tennis player on what date?