The 40-year-old Williams registered her first singles victory of the season as she beat Nuria Parrizas-Diaz 6-3, 6-4. However, her older sister suffered defeat at the hands of Jil Teichmann.
Rain played spoilsport in the men's singles, as several matches were halted while Andy Murray's match against Taylor Fritz was postponed to Tuesday.
In the few matches that took place, Stan Wawrinka lost to Emil Ruusuvuori while Jenson Brooksby booked his place in the second round by defeating Alexander Bublik in straight sets.
Let's take a look at the results from Day 1 of the Canadian Open.
Serena Williams beats Nuria Parrizas-Diaz to register first singles win since June 2021
Serena Williams booked her place in the second round of the Canadian Open with a straight-sets win over Spain's Nuria Parrizas-Diaz in two hours and one minute.
Both players exchanged early breaks but the 40-year-old made a decisive one in the eighth game to go 5-3 up. She then held her serve to take the first set 6-3.
The second set was tightly contested with both players looking to break the other's serve. Williams saved four break points in the eighth game to level the score at 4-4. She then broke Parrizas-Diaz before holding her serve to win the match and seal her place in the second round.
Serena Williams will face either Belinda Bencic or Tereza Martincova next.
Jil Teichmann eliminates Venus Williams
Unlike her sister, Venus Williams could not win her opening round match, falling to World No. 21 Jil Teichmann.
The Swiss started the match strongly as she broke the former World No. 1 twice in the opening set, eventually winning it 6-2. The second set saw both players exchanging breaks before Teichmann took three out of the last four games to win the set and the match.
The Swiss will next face World No. 2 Anett Kontaveit for a place in the last 16 of the Canadian Open.
Stan Wawrinka goes down fighting to Emil Ruusuvuori
Stan Wawrinka suffered an opening-round elimination at the Canadian Open after losing to Emil Ruusuvuori despite putting up a tough fight.
Ruusuvuori made an early break in the first set which turned out to be decisive as he won the set 6-3. Wawrinka was trailing by a break in the second set but he bounced back and went on to win the set 6-3 to force the match into a decider.
Both players held their serve well in the final set before Ruusuvuori made a crucial break to go 4-2 up. Wawrinka had a breakpoint in the last game, which the Finn saved and then held his serve to win the set and the match.
Ruusuvuori will face Hubert Hurkacz in the second round.
Canadian Open 2022: Day 1 at a glance
Men's Singles first round:
Emil Ruusuvuori def. (PR) Stan Wawrinka 6-3, 3-6, 6-3
Karen Khachanov def. Francisco Cerundolo 7-6(4), 5-7, 6-3
Alex Molcan def. Mackenzie McDonald 7-6(1), 6-4
Jenson Brooksby def. Alexander Bublik 6-2, 6-3
Women's Singles first round:
(15) Simona Halep def. (LL) Donna Vekic 6-0, 6-2
(PR) Serena Williams def. (LL) Nuria Parrizas- Diaz 6-3, 6-4
Sloane Stephens def. (PR) Sofia Kenin 6-2, 6-7(5), 7-5
(13) Leylah Fernandez def. (Q) Storm Sanders 6-4, 6-7(2), 6-3
Elena Rybakina def. (Q) Marie Bouzkova 7-5, 6-7(3), 6-1
(14) Karolina Pliskova def. Barbora Krejcikova 6-3, 6-4
Alize Cornet def. Caroline Garcia 3-6, 6-3, 6-3
Alison Riske def. Petra Kvitova 6-2, 4-6, 6-3
Beatriz Haddad Maia def. Martina Trevisan 6-2, 2-6, 6-2
(16) Jelena Ostapenko def. Anhelina Kalinina 6-4, 6-2
(Q) Asia Muhammad def. Madison Keys 7-6(5), 6-4
Yulia Putintseva def. (WC) Katherine Sebov 3-6, 6-2, 7-5
Jil Teichmann def. (WC) Venus Williams 6-2, 6-3