United manager Ian Walker was disappointed with the result but praised his side who had no substitutes available due to injuries and illness.
"It is a big knockback for us but we had five of our starting line up out and at the end of the game that really showed," said Walker.
"We could have got a result but we just ran out of steam. I am pleased with the boys though because they played with a lot of heart.
"Dayne of course impressed and he was very clinical in front of goal, it's just a shame about the result."
Willemsen has netted 11 goals this season in seven appearances for United, scoring all of his six penalties.
And the pick of his strikes on Saturday was his opening goal just three minutes into the game when he skilfully beat two players in the box, from a Jason Walker drag back, and slotted home.
"It's a fine run of form and I hope he continues finding the back of the net," said Walker.
"He is incredibly talented and it was a really special goal. Jason worked his magic on the wing but Dayne had a lot to do. It was a quality finish, from a quality player."
The result sees Matamata leap frog United into second place and Walker's side must pick themselves up quickly as they face tough opposition next weekend when they host fourth place Cambridge FC.
"It's a really big game and if we want to stay in the hunt at the top of the table we need a result," Walker said.
"The players just need to put this result behind them and get their heads in the game for next Saturday.
"Hopefully we'll have returning players and a stronger side to make a contest out of the match."
Results:
WaiBOP Premiership
SGS Papamoa 1 Melville United 1
Tauranga City 0 Old Blues 4
Waikato Stud Matamata Swifts 4 Rotorua United 3
Waikato Unicol AFC 2 Katikati FC 0
Cambridge FC v Tauranga Boys College - Postponed
Ngongotaha AFC v Otumoetai FC - Postponed
WaiBOP Championship
Cambridge FC Reserves 0 Te Awamutu Edmond Judd 3
Matamata Swift Reserves 4 Hamilton Wanderers 2
Waikato Unicol 0 Tokoroa AFC 0
West Hamilton 5 Whakatane Town 1
Ngongotaha Reserves v Otumoetai Reserves - Postponed
Taupo AFC v Old Blues Reserves - Postponed