Tottenham face an arduous task to keep their Champions League hopes alive after Ajax secured a crucial advantage in the semi-final first leg.Donny van de Beek's 15th-minute goal, steered cleverly past Hugo Lloris from close range, put Erik ten Hag's exciting young side firmly in the driving seat going into the second leg in Amsterdam next Wednesday.Spurs struggled to overcome the absence of forwards Harry Kane and Son Heung-min - injured and suspended respectively - and their job was made even harder by the loss of defender Jan Vertonghen after he suffered a facial injury in the first half.Vertonghen's problem raised questions about player welfare after he was allowed to continue, albeit for only a few seconds, when he was clearly badly shaken up after an aerial collision involving team-mate Toby Alderweireld and Ajax goalkeeper Andre Onana. He eventually had to be supported by two members of the Spurs staff as he went off.Tottenham tried to force the pace after the break with plenty of possession, but it was Ajax who came close to adding a second when David Neres struck the inside of the post with Lloris beaten.Ajax held on to their lead in relative comfort and it will need a stirring Spurs comeback to prevent the Dutch side facing either Barcelona or Liverpool in the final on 1 June.