"We always knew it was going to be a tough challenge to come to Bloemfontein and play against a very good Cheetahs team," Spies said.
"I am very proud of the boys for their performance."
With three games left after the June test window, the Bulls needed a strong performance against the Cheetahs, and they delivered.
With good attacking play, solid defence and a 15-point contribution by Springbok flyhalf Morne Steyn, the Bulls consolidated their place at the top of the South African conference.
"We've worked hard to obtain a mixture of youth and experience in the side," Spies said.
"We've got a pretty good squad and we have been lucky with regards to injuries.
"When injuries did occur, we could fill the gap with another quality player. It took hard work and we are very happy with the result."
The Cheetahs, enjoying their best Super Rugby season since becoming a franchise, challenged the Bulls in every area, particularly at the scrums.
The Bloemfontein outfit settled in fifth place overall on the log, well in contention for a spot in the knockout stages.
"We knew before the game that we needed to get out of our own half but, especially in the first half, we couldn't do that," said Cheetahs skipper Adriaan Strauss.
"We also gave the Bulls the opportunity to run at us the whole game. We made it quite tough on ourselves.
"We are an attacking side and we couldn't get into their half and start running with the ball."
Some ill discipline also cost the hosts, with Steyn's accurate kicking boot punishing the Cheetahs for any offences within striking distance.
"I must admit that a lack of discipline at crucial moments did cost us a fair bit," Strauss said.
"Our lineouts were also a big problem as we couldn't get any platform to play from there, especially when we got the lineout in their half. We couldn't capitalise on that.
"There are some positives, but in general we are very disappointed with the outcome."