It's the culmination of some solid results for the 32 year old who made the quarter finals at Wimbledon last month and reached a career high of 34 in the rankings. (He's currently 37th).
However playing with a doubles partner for the first time is never easy and the experienced combination of Venus and Klaasen took control in the 9th game of the first set.
Games had gone with serve until 4-4 before Sitak found himself under pressure. He missed a volley to bring up a break point and then hit a forehand wide and Venus served out the opening set in 30 minutes.
Tsitsipas dropped serve in the opening game of the second set and after Klaasen held for 2-love, Klaasen missed a short put away on deciding point on Sitak's serve that would have given the South African and Kiwi a 3-0 lead.
It didn't matter however as Venus and Klaasen served superbly throughout the remainder of the set and never gave their opponents a look in. They lost just one point on serve in the second set.
Venus is defending no points this week after losing in the first round in Canada last year when the tournament was held in Montreal. He has a lot of points to defend from next week's Cincinnati Masters after making the semi finals last year so is looking for a good run in Toronto to continue his push with Klaasen to make the season ending ATP World Tour finals in London.
Venus and Klaasen are currently 6th in the race with the top eight teams at the end of the season qualifying for the lucrative finale where Venus debuted as French Open champion last year with Ryan Harrison and reached the semi finals.