Netherlands were dominant but wasteful in the first half at the Al Thumama Stadium, and lacked threat for much of the second half until Gakpo nodded in a deep cross on 85 minutes. Klassen added a second in the final minute of the match to ensure Louis van Gaal’s side enjoyed a positive start to their campaign.
Netherlands were playing in their first World Cup match since 2014, having failed to qualify for the tournament’s previous edition in Russia in 2018.
Senegal, who won last year’s Africa Cup of Nations, flew to Qatar with high expectations but suffered a major blow last week when star Sadio Mane was ruled out for the World Cup with a leg injury, and his absence showed.
Netherlands dominated the first-half and provided regular pressure around the Senegal penalty area, but were poor in front of goal. The Dutch side squandered a glorious opportunity to take the lead on six minutes as Gakpo squared the ball to teammate Steven Bergwijn, whose attempt was blocked.
Another chance came on 18 minutes when midfielder Frenkie de Jong found himself in plenty of space and with just opposing goalkeeper Edouard Mendy to beat, but he took too long to get a shot away and the move broke down.
Senegal, meanwhile, looked pedestrian in attack as they badly missed the threat of Mane.
Netherlands looked less in control in a largely dull second half, and Senegal enjoyed a pair of promising opportunities late on as Dutch goalkeeper Andries Noppert — making his international debut — was forced into a save from Idrissa Gueye, and then from Ismaila Sarr.
But Netherlands found their clinical touch late on. Gakpo broke the deadlock when he met a delightful deep cross from De Jong and headed Netherlands into the lead.
Referee Wilton Pereira Sampaio signalled for eight minutes added time, and Netherlands used it well as Memphis Depay’s shot forced Mendy to parry to Klaassen, who provided a comfortable tap-in.
Dutch captain Virgil van Dijk did not wear the OneLove anti-discrimination armband, with FIFA threatening to impose sanctions of a yellow card for any player seen wearing it.
When asked after the match what he made of the issue, Van Gaal said: “I no longer want to answer political questions. This is the time to talk about football and not political issues. One thing is clear — we are not going to wear the armband if we’re getting a yellow card. I think we need to question whether this a correct action on the part of FIFA and you can answer that question.
“Virgil van Dijk after one match is to be given a yellow card! We are here to become World Champions and that is our purpose, which is why we’re not wearing the armband. All of us need to ask ourselves whether this is a proper action on the part of FIFA — that is crystal clear.”