Yannick was a revelation last season in Lisbon and has also impressed so far this term in the Portuguese Superliga and the Champions League.
"The agreement is excellent and now I only want to focus on playing for my club," he said.
Fernandes future unclear as Man Utd move ‘appears unlikely’
Manchester United and Barcelona had been linked with the Guinea-Bissau born Portugal international.
Liverpool held as Man Utd target Fernandes stars for Sporting
The Warm-Up: Get Welbz soon