Three thoughts: Serena survives challenge vs. Wozniacki at Rogers Cup

Serena Williams survived a stern challenge from her good friend Caroline Wozniacki, breaking in the final game of the last two sets to complete a 4-6, 7-5, 7-5 comeback win in the Rogers Cup quarterfinals.

Three thoughts on Serena's hard-fought win:

Serena won without her A-game: This has been the predominant refrain since her post-Wimbledon return. Her serve still isn't clicking, and she has struggled to find her range early in matches. Slow starts put her up against the wall in three consecutive matches last week in Stanford. But just as she was able to steady herself and not panic en route to the Stanford title, Serena kept her composure today against an opponent who was playing some of her best tennis.

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Over the course of the two-hour and 40-minute match Serena fired 53 unforced errors, most of them landing long (she also hit 51 winners). She started the match serving ok but her first serve deserted her in the final set, where she went from serving at 72 percent in the first two sets to 39 percent in the third. Any leads she built in the decisive set she quickly relinquished, getting broken twice. Finally, just as she did to steal the second set, Williams took advantage of a poor service game from Wozniacki at 5-6 and broke for the victory.

Wozniacki still hasn't beaten a top 15 player this year: However, if she consistently played the way she did today, she would have beaten 14 of the top 15 players. After she won her first title of the season at the Istanbul Cup last month, I wrote that we still needed to take a wait-and-see approach to Wozniacki's game. Though she's shown clear signs of a turnaround since the French Open, she hasn't been truly tested, and it was difficult to gauge how her improved form would fare against the game's elite.

But this was the best Wozniacki has played since she beat Serena 6-4, 6-4 in Miami in 2012. There was aggressive intent in her decision-making. She came to the net, she went for winners, she pushed herself inside the court instead of dropping back. This was a loss to build on.

An all-Williams semifinal looms: Venus Williams beat Carla Suarez Navarro 4-6, 6-2, 6-3 in the quarterfinals, and she'll face sister Serena in Saturday's semifinal. If you didn't get a chance to see Venus' incredible three set win over No. 8 Angelique Kerber last night, take a look at the highlights. This is the first meeting between the Williams sisters this year, and their first since Charleston last year, where Serena won 6-1, 6-2.