The roof was closed over the court due to rain and the ball flew through the air in the humid conditions with the altitude also making it must faster than most of the tournaments on the European red clay.
Venus and Klaasen gradually found their rhythm on serve in the second set but still saved four break points. They continued to struggle on their opponents serve before turning things around late in the set.
Leading 6-5 they hey produced a superb return game to break Marc Lopez to love and take the second set 7-5. It was the first break points they had conjured up in the match and took the momentum into the first to 10 points match tie break.
"We hadn't been in many return games so were aggressive and managed to connect on a few of the returns which put us in a great position to go into the match tiebreak." Venus said.
Venus and Klaasen got out to an early lead and Venus served for the match at 9-7. While he couldn't close it out on serve, Venus and Klaasen won the next point to secure their place in the second round.
It's a significant result for the Kiwi who has no points to defend before the defence of his French Open title later this month. With 1000 points on offer for the winners this week and next week at the Rome Masters, Venus is looking to minimise the effect of any early exit in Paris where he has 2000 points to defend from his Roland Garros triumph last year.
Venus and Klaasen will face the 8th seeds India's Rohan Bopanna and Frenchman Edouard Roger-Vasselin in the second round in Madrid.